datatype from has_format

[dsavchenko]

Currently, parse_format_annotations() adds restriction on has_format directly to the constructed class, not following the way proposed in oda-hub/ontology#35
Two possible adaptations:

1. Instead, when processing annotations, construct superclasses with both restrictions
2. Modify _get_datatype_restriction() to infer datatype from restriction on has_format. Will require to express format-datatype correspondence in ontology.