Sanitize the irfgen folder

[lucabaldini]

While splitting the code for the generation of the response files out of the older ixpeobssim private repository I did notice that the original cut between the modules that were released and those that were not was suboptimal.

More specifically, in order to decouple things in a sensible way, I ended up adding to the new (private) ixpeirfgen repository the following files

ixpeirfgen/auxiliary.py
ixpeirfgen/base.py
ixpeirfgen/data/du/
ixpeirfgen/du.py
ixpeirfgen/gpd.py
ixpeirfgen/mma.py

Bottomline: we currently have significant code duplication between ixpeobssim and ixpeirfgen, that would be good to reduce in the future.

I am under the impression that some of the aforementioned files (e.g., base.py) could be just removed from ixpeobssim, and others could be stripped down to the bare minimum.