Replaced Analytical1DLikelihood._infer_parameters_from_function with utils function that is also used in the waveform generator

Merge branch '157-simplify-likelihoods' into 166-implement-caching-in-waveformgenerator-__parameters_from_source_model

Moved `infer_parameters_from_function` function here because it is used both in `waveform_generator` and `likelihood`

Refactored some old tests to make them separate and test one feature at a time

Created two properties for the rate and the sumlogfactorial respectively. Simplified log_likelihood.