Optionally use class docstrings for rdfs:comment on DataObject classes

[mwatts15]

In many cases, a DataObject sub-class' definition's docstring may also serve as a rdfs:comment for the corresponding RDF type. This issue is to add an attribute to ContextMappedClass, the DataObject meta-class, which will cause the rdf_type_object for the class to have an rdfs:comment with value equal to the class docstring. The setting for this attribute should not be inherited by sub-classes.