Custom RDF subclassing

[mpsonntag]

Currently only entries in file odml.resources.section_subclasses.yaml will be exported to RDF.

• Users should be able to provide an additional map of Section.type to RDF Subclass name. If provided, this map is used in addition to the default map.
• Users should be warned when a Section.type in the custom mapping overwrites a default one with the same Section.type, but the custom one should overrule the default entry.