suspension thermal noise simplification

new suspension_thermal() function for calculating thermal noise of a
single suspended optic.

ifo SuspensionThermal Noise class modified to handle specific case of
Fabry-Perot Michelson.

gwinc/ifo/noises.py

@@ -103,7 +103,8 @@ class SuspensionThermal(nb.Noise):
     )
 
     def calc(self):
-        return noise.suspensionthermal.susptherm(self.freq, self.ifo)
+        n = noise.suspensionthermal.suspension_thermal(self.freq, self.ifo.Suspension)
+        return n * 4 * self.ifo.gwinc.dhdl_sqr
 
 
 class CoatingBrownian(nb.Noise):

gwinc/noise/suspensionthermal.py

+'''Functions to calculate suspension thermal noise
+
+'''
 from __future__ import division
-from numpy import pi, imag, zeros
 import numpy as np
+from numpy import pi, imag, zeros
 
 from .. import const
 
@@ -8,14 +11,17 @@ from .. import const
 def suspension_thermal(f, sus):
     """Suspention thermal noise for a single suspended test mass
 
-    'Temp' must either be set for each stage individually, or globally
-    in ifo.Suspension.Temp.  The latter will be preferred if
-    specified, so if you wish to use per-stage tempurature you must
-    remove Suspension.Temp.
+    :f: frequency array in Hz
+    :sus: gwinc suspension structure
+
+    :returns: displacement noise power spectrum at :f:
+
+    :Temp: must either be set for each stage individually, or globally
+    in :sus.Temp:.  The latter will be preferred if specified, so if
+    you wish to use per-stage tempurature you must remove :sus.Temp:.
 
     Assumes suspension transfer functions and V-H coupling have been
-    pre-calculated and populated into the relevant `ifo` struct
-    fields.
+    pre-calculated and populated into the relevant `sus` fields.
 
     """
     # Assign Physical Constants
@@ -74,7 +80,4 @@ def suspension_thermal(f, sus):
             w = 2*pi*f
             noise += 4 * kB * stage.Temp * abs(imag(dxdF[n, :])) / w
 
-    # 4 masses, turn into gravitational wave strain
-    noise *= 4 * ifo.gwinc.dhdl_sqr
-
     return np.squeeze(noise)