No more precomp

This finally gets rid of the whole precompIFO function. It does this by breaking up precomp into various smaller functions, moving them into the common IFO noise definition module, and using them appropriately when needed. This does result in some of the functions being called multiple times in the full budget calculation for the aLIGO-like budgets, but the cost should be minor given the convenience of getting rid of the whole precomp thing, thereby opening up everything to non-aLIGO-like configurations.

Even though some functions are calculated multiple times, we're still orders of magnitude faster than matgwinc, so... Should still revisit down the line though.

closes #40 (closed)

gwinc/nb.py

@@ -52,15 +52,21 @@ class BudgetItem:
 
     ##########
 
-    def __init__(self, freq, **kwargs):
+    def __init__(self, freq=None, **kwargs):
         """Initialize budget item.
 
-        Primary argument is the evaluation frequency array.  Any
-        keyword arguments provided are simple written as attribute
-        variables in the initialized object.
+        The primary argument should be the evaluation frequency array.
+        If it is not provided, then it is assumed to be a pre-defined
+        attribute of the BudgetItem class.  Any keyword arguments
+        provided are simple written as attribute variables in the
+        initialized object.
 
         """
-        self.__freq = freq
+        if freq is not None:
+            assert isinstance(freq, np.ndarray)
+            self.freq = freq
+        elif not hasattr(self, 'freq'):
+            raise AttributeError("Frequency array not provided or defined.")
         for key, val in kwargs.items():
             setattr(self, key, val)
 
@@ -76,11 +82,6 @@ class BudgetItem:
             self.name,
         )
 
-    @property
-    def freq(self):
-        """Evaluation frequency array supplied at initialization."""
-        return self.__freq
-
     def interpolate(self, freq, data):
         """Interpolate data to the evaluation frequencies.