Tiny cosmetic change to "getSource" in "EntityWithSources".
Modified "Source" so that all (optional) getter of entities return empty entities if field missing.

"getSource" now checks if source is in group and returns empty entity if not.
Tiny bugfix to "EntityWithSources".
Modified (optional) getter in "File" to check if id exists and return empty object otherwise.

Modified "getSection" and "getBlock" to check if id exists and return empty object otherwise.
Modified "data" and "linkType" getters in "Representation" to check whether fields exist.
Minor cosmetic changes to Section.
Minor corrections to "getReference" in "DataTag".
Modified (optional) getter of entities in "SimpleTag" to return empty entity if field missing.

Modified "getRepresentation" to return empty entity if field not found and modified "getReference" to check for ownership (id in group) and not just existence (id in block).