Initial work on XDR parser

[fabdrol]

This PR implements an initial implementation for an XDR parser that uses the definitions format from xdr-parser-plugin. The implementation is different however, as that plugin has several issues IMO (e.g. require statements within function bodies).

In order to finish this PR, it would make sense (to me) to internalise the dictionary of XDR sentences rather tha pulling it from xdr-parser-plugin, and expanding on the definitions in there. Also, it would make sense to make the expression dependent on the unit (RN I'm simply checking if the units match).