Conversion update

Makes some calls to the conversion module more explicit by returning mutated lists rather than mutating the handed over list. I also extracted a bunch of the conversion logic in convert_to_lal_binary_black_hole_parameters, which allows the user to use conversion logic between component masses, mass ratio, total mass, symmetric mass ratio and the chirp mass.

I added some tests for conversion.py.

Additionally, I changed the implementation of parameters and non_standard_sampling_parameter_keys in GravitationalWaveTransient so that uses a property and setters to refer to it's WaveformGenerator's attributes. I reckon this makes the implementation a bit safer.