Add debug output for GeoModelXml copy number levels

As discussed in #37, the CNL_N are one of the more complicated things to get right when developing geometries in GeoModelXml. This MR intends to add some optional debug output which should give useful information when trying to work out the mapping of copy number levels to structures.

Suggestions for improvements (especially for how to pass optional parameters to GeoModelXml through the plugin) are very welcome.

FYI @ncalace