Extra likelihood kwargs is a string and is not converted to dict

extra_likelihood_kwargs string read in from the config is not converted to dict internally.

@apratim.ganguly found this error while doing a parallel_bilby run. An easy fix is to use bilby_pipe.utils.convert_string_to_dict at https://git.ligo.org/lscsoft/parallel_bilby/-/blob/master/parallel_bilby/analysis.py#L130

I have fixed the issue on this branch](https://git.ligo.org/aditya.vijaykumar/parallel_bilby/-/commits/fix_extra_likelihood_kwargs) and am happy to start an MR if the maintainers agree 😄