fix javadoc param typo
adjust XML mapping files
introduce String to RString maps into model
create mapper between String and RString map values
fill in new OMSC methods
adjust and add to tests
Merge 'mark/dsl-map-type-joshmoore' into dsl-map-type
Fix flake8 whitespace in test
Update test_mapannotation.py for rstring

@mtbc's work moved the config (and in fact all map columns)
to `Map<String, RString>`. This defines the logic that is
expected for regular entries, empty strings, and Nones (nulls).