Make value of maps NON-NULL

Since Ice converts a null string to an empty
string, this change as it stands will store
empty strings in the DB. An RType will be
required to prevent that.
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).
rename GenericSource to GenericExcitationSource
Use mapping code that makes null-handling more obvious.
fix integration test to show nulling behavior