Added custom type for multi-line string to types.py
Added custom representer and dumper for multi-line-strings
Updated EventClassSpec(Params) and EventClassMappingSpec(Params) to
use multi-line strings for relevant attributes (description, transform,
example, explanation, resolution)
Added test_event_class_export.py unit test
Updated documentation with notes about multi-line handling and about
overriding existing properties