Interface simplifications

A series of interface improvements

BREAKS THE CURRENT INTERFACE, but I think the improvements/simplifications are worth it:

• new load_budget() convenience function returns only Budget class, with ifo pre-set as class attribute.
• clean ups to gwinc() convenience function
• add Budget.run() method as simple interface to run all usual methods for common use case.

gwinc/__init__.py

@@ -112,6 +112,7 @@ def gwinc(freq, ifo, source=None, plot=False, PRfixed=True):
     # FIXME: how do we allow adding arbitrary addtional noise sources
     # from just ifo description, without having to specify full budget
     Budget = load_budget('aLIGO')
+    ifo = precompIFO(freq, Budget.ifo)
     traces = Budget(freq, ifo=ifo).run()
     plot_style = getattr(Budget, 'plot_style', {})