Added new argument to write json from mfile, added execution of argument to post processing

[grmtrkngtn]

It is useful to have PROCESS dump the mfile to json for UQ work. I have added an argument to do this which is executed in postprocessing

[timothy-nunn]

Please could you add an integration test for this (maybe with a once through input file?):

• Does running with this command produce a JSON file at the expected location with the expected name?
• Does the JSON file contain valid JSON?
• Does the JSON contain some outputs we would expect?

[timothy-nunn]

Suggested change

	mfile_str = str(mfile.resolve())
	mfile_str = str(mfile_path.resolve())

[grmtrkngtn]

    if self.args.plot:
        # Check mfile exists, then plot
        mfile_path = Path(self.args.mfile)
        mfile_str = str(mfile_path.resolve())

[timothy-nunn]

Update the help message slightly. Where does it write the JSON file, and with what name?

[grmtrkngtn]

    parser.add_argument(
        "-mj",
        "--mfilejson",
        action="store_true",
        help="Produce a filled json from --mfile arg in working dir",
    )

Modified like this

[grmtrkngtn]

I addressed the changes, let me know if further improvements are needed.