Errase `generic.py` in `theory`

[marina-ricci]

After discussion with @m-aguena, we don't see the purpose of the current generic.py in the theory module. There is only simple arithmetic operations in it. See conversation in issue #439. If people agree, let put generic.py on retirement, and maybe resuscitate later if we need it.

[combet]

Hi - just trying to catch up with this discussion. What is the status of this issue? @marina-ricci @m-aguena.