Add full support for CDF

[GillesDuvert]

(naive remark: not the same as NetCDF) This is an important scientific format (even if I hear of it just now :blush: ) If we had DLM added to GDL --- adding e.g., CDF would 'just' need to write the appropriate .dlm file plus all the needed C code for interfacing the DLM library (which is not so obvious if, as for CDF, the number of commands is large). At the moment, doing an intermediate solution as we did for GraphicsMagick seems more easy. Another solution for DLM is to not try to mimick IDL and just conceive something simple using SWIG or equivalent. This would avoid a lot of trouble IMHO.

[slayoo]

see also #87