Simplify parameter logic

This build on !36 (merged) so should be merged after (if it is to be merged)

The Sampler and fill_prior access sampling parameters via the waveform_generator attribute of the likelihood. This therefore places a requirement that the likelihood have a waveform_generator.

To remove this requirement, let's require the likelihood to have a parameters instance and access them this way. For our likelihood, this is as simple as setting them from the waveform_generator.

Further, this will make it easier in future to write different likelihood which may not need a waveform generator, but can handle those issues internally.