Clean up the logic of fill_priors and close #58

• Removes the 'bad_keys' part of the logic. I believe this was essentially adding the keys which where not given in the prior and that had no default prior, and then removing them at the end.
• Change the logging output slightly: now the only warning that is given is when no default prior is known. Other output at info level
• Add in what the waveform_generator knows about missing keys (useful for diagnosis
• Formatting of the docstring

Note - in closing #58 (closed), it doesn't now explicitly warn the user. This was done on purpose as the only way to do this would be to either set geocent_time as somehow special and requiring a value, or insisting that all values are not None. But in future, we may want to have parameters that can be None. So instead, to debug when you forgot to set geocent_time, it tells you that it is set to None.

mentioned in merge request !37 (merged)

Looks good, I'll merge.

mentioned in commit e6d489c6

merged