neo_doxygen: Introduce a tool to import a Doxygen model.

This tool converts the XML output generated by Doxygen to a Neo4j graph suitable for `neo.nit`.

In summary, the following is imported:
* Namespaces
* Files, generating one module for each file-namespace pair
* Classes, with inheritances
* Members, with signatures and redefinition relationships (when Doxygen provide them)
* Documentation (without formatting for the moment)

Concerning language-specific support, only some Java keywords are correctly interpreted at this point.

Note: `/cotrib/neo_doxygen/gen-all.sh` and `/cotrib/neo_doxygen/gen-once.sh` call `nx`, so they require PR #744.

@Morriar: The program was ready for Friday, but the history was a mess.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
Tant qu’à mettre une déclaration qui est optionnelle, pourquoi ne pas être précis?