This adds a Struct() class that mimics a MATLAB struct by storing values as
class attributes, but also includes methods for converting to/from dicts,
yaml, and mat_struct objects.

A convenience function load_ifo() is included to load an ifo definition
either from a .yaml or .mat file or from an included <ifo>.yaml.

An 'aLIGO.yaml' definition is included.

no functional change

All noise calculation is moved to noise_calc() function.  Plotting is moved
to plot_noise() function.  The gwinc() function is now just a wrapper on
these two functions with some additional status reporting.

Reproduces the Voyager noise curves but still needs testing with other IFO models.