Migrate group and qmap code from nanite to afmformats

[paulmueller]

...since the corresponding metadata are defined in afmformats anyway.

Also, we should change submodule names:

• afm_data -> OK
• afm_fdist -> mod_force_distance (subclasses from afm_data, ~afm_group, afm_qmap~)
• new: afm_group (from nanite)
• new: afm_qmap (from nanite)

~Also, make sure that class names correspond to the "mod" name. I.e. ForceDistanceQMap~ this should be done in nanite.