This is now commensurate with what matlab gwinc does, and makes the IFO
structures directly comparable without any hacky conversions.

This makes it more compatible with matlab, allowing the resultant object
to be saved to a .mat and imported in matlab in the same format as matlab
gwinc ifo structures.

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.

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