simplify noise calculation functions
This series simplifies the noise calculation functions to return displacement noise spectrum for simplest logical element, e.g. a single (suspended) test mass, removing any assumptions about interferometer topology.
Corrections for IFO topology are now handled in the ifo.noises Noise BudgetItem classes.
This makes the noise calculation functions more generally usable for arbitrary experimental setups, not just for aLIGO-like detectors. I therefore think this is an important improvement for pygwinc.