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