Doc string enhancements

[lpicci96]

Doc strings should be improved for greater clarity:

• remove_by_provenance: add parameter descriptions to the docstring. Add information to the docstring - e.g. does the provenance name in the sources section also get removes or is it only the variables and the data files?
• remove_provenance - Needs parameter description
• remove_by_source needs a parameter description. Same issue as remove_by_provenance
• remove_source needs parameter description
• merge_configs_from_directory has a missing parameter not described in the docstring