xsd-fu template documentation and metadatastore bugfixing

This PR:

- Adds type information to metadata store method signature data to allow documenting the store APIs
- Removes unwanted methods from DummyMetadata (template bug)
- Removes unnecessary and unused arguments from one MetadataRetrieve getter variant (template bug)
- Removes unneeded header includes from the metadata headers
- Adds doxygen comments for every model and metadata class and methods, including all method arguments
- Fixes some broken doxygen comments

With this PR applied, there is complete doxygen API coverage for all the C++ code.

Note that the template bugfixes do change the API for both C++ and Java.  The DummyMetadata methods will never have been used (they aren't part of the MetadataStore interface).  The argument removal does not affect any code in-tree, but there may be potential external users of these methods (though unlikely).