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