Suspension thermal noise sub-budgets

Adds suspension thermal noise sub-budgets. This addresses 1/3 of #51 (closed). Suspension thermal noise can now be broken up into horizontal and vertical noise of each of the suspension stages. With one exception, which does not change the total noise, the calculation is the same as the current calculation but the contributions of each stage and direction are tracked separately.

In the current calculation the horizontal and vertical spring constants of each stage are broken up into an upper and lower joint and are stored separately. The total suspension thermal noise is then calculated by adding the contributions of each of these joints using the appropriate temperature of each joint. Instead of adding the contributions of each joint all at once, this MR makes it possible to track each stage with separate Noise classes and add them all to a sub-budget.

The noise for each stage is the sum of the noise from that stage's lower joint and the noise from the upper joint of the stage below. In the case of the top mass, the contribution from the upper joint is also included. In the current calculation, for the upper stages, the imaginary part of the horizontal spring constant is equally distributed between the upper and lower joints while it is only attributed to the upper joint for the test mass. The one change this MR makes to the calculation itself is to distribute this equally between the top and bottom joints as is done for the upper stages.

The imaginary part of the vertical spring constant is only attributed to the upper joint of each stage. The blade springs are the only contribution to the vertical spring constant for the upper stages while the lower stage also includes the continuum fibers or ribbons. It's unclear how to break this up into an upper and lower stage as is done in figure 14 of https://arxiv.org/abs/2001.11173, so this MR does not have vertical noise of the test mass. (The noise is included, it's just attributed entirely to the PUM.)

The sub-budgets are only implemented for the Cosmic Explorer interferometers since these are the only ones that have any other sub-budgets. The CE1 suspension thermal noise budget is shown below as an example.

gwinc/__init__.py

@@ -14,6 +14,31 @@ from .plot import plot_noise
 logger = logging.getLogger('gwinc')
 
 
+DEFAULT_FREQ = '5:3000:6000'
+
+
+def freq_from_spec(spec=None):
+    """logarithmicly spaced frequency array, based on specification string
+
+    Specification string should be of form 'START:[NPOINTS:]STOP'.  If
+    `spec` is an array, then the array is returned as-is, and if it's
+    None the DEFAULT_FREQ specification is used.
+
+    """
+    if isinstance(spec, np.ndarray):
+        return spec
+    elif spec is None:
+        spec = DEFAULT_FREQ
+    fspec = spec.split(':')
+    if len(fspec) == 2:
+        fspec = fspec[0], DEFAULT_FREQ.split(':')[1], fspec[1]
+    return np.logspace(
+        np.log10(float(fspec[0])),
+        np.log10(float(fspec[2])),
+        int(fspec[1]),
+    )
+
+
 def load_module(name_or_path):
     """Load module from name or path.
 
@@ -43,19 +68,24 @@ def load_budget(name_or_path, freq=None):
 
     Accepts either the name of a built-in canonical budget (see
     gwinc.IFOS), the path to a budget package (directory) or module
-    (ending in .py), or the path to an IFO struct definition file (see
+    (ending in .py), or the path to an IFO Struct definition file (see
     gwinc.Struct).
 
     If the budget is a package directory which includes an 'ifo.yaml'
     file the ifo Struct will be loaded from that file and assigned to
-    the budget.ifo attribute.  If a struct definition file is provided
-    the base aLIGO budget definition and ifo will be assumed.
+    the budget.ifo attribute.  If a Struct definition file is provided
+    the base aLIGO budget definition will be assumed.
+
+    Returns an instantiated Budget object.  If a frequency array or
+    frequency specification string (see `freq_from_spec()`) is
+    provided, the budget will be instantiated with the provided array.
+    If a frequency array is not provided and the Budget class
+    definition includes a `freq` attribute defining either an array or
+    specification string, then that array will be used.  Otherwise a
+    default array will be provided (see DEFAULT_FREQ).
 
-    Returns a Budget object instantiated with the provided frequency
-    array (if specified), and with any included ifo assigned as an
-    object attribute.  If a frequency array is not provided and the
-    budget class does not define it's own, the frequency array must be
-    provided at budget update() or run() time.
+    Any included ifo will be assigned as an attribute to the returned
+    Budget object.
 
     """
     ifo = None
@@ -85,6 +115,9 @@ def load_budget(name_or_path, freq=None):
     logger.info("loading module {}...".format(modname))
     mod, modpath = load_module(modname)
     Budget = getattr(mod, bname)
+    if freq is None:
+        freq = getattr(Budget, 'freq', None)
+    freq = freq_from_spec(freq)
     ifopath = os.path.join(modpath, 'ifo.yaml')
     if not ifo and os.path.exists(ifopath):
         ifo = Struct.from_file(ifopath)