Refactor parset.py

Refactor parset.py. All the logic to read a Rapthor parset file and handle the different options were all done in a single parset_read function. A new Parset class was created that handles reading of parset files, validation of its contents and the conversion to a dict. The remaining part of the logic in the original parset_read function dealt with preparing the runtime environment, e.g., by creating a working directory, checking for the presence of input MS files, etc. That part has been retained. You can still debate if parset_read is the proper name for such a function.

Closes RAP-406