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.

@Morriar: The program was ready for Friday, but the history was a mess.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
pour ce tas de services, est-ce que faire des abort seraient plus correct que retourner des trucs qu'ont pas forcément du sens? Ca permettrai de détecter des trucs louches plus tôt.

Pas plus que quiconque j'aime les fonctions partielles mais des fois c'est moins sale que retourner un truc approximatif.