Add --force_replace option and support for a new R action in the spec file

[r3m0chop]

To initially only be user-configurable to a single value for any of the METADATA tags that are defined to a new value of R in the spec file (e.g., create a new dicom_anon_spec_files/ext_keep_study_and_series_descs_replacePatientNameID.dat) using a new option of:

    parser.add_argument('-f', '--force_replace', type=str, default='REDACTED',
                        help='Replacement string for any fields with in-house rule R. Defaults to REDACTED')

And to eventually possibly even support a user-supplied comma-delimited set of key/value pairs in the --force_replace, allowing a different value for each such tag. For now, though, it will be sufficient for this option to support the forcing of values for both the PatientsName and the PatientID (e.g., if so specified in the spec file) to the same user-supplied value.