[cctbxbb] from __future__ import division has become compulsory

[Luc Bourhis]

Dear cctbx developers,

as discussed here a few days ago, it has been decided that every single Python module must feature
from __future__ import division
A mega commit of mine has just taken care of existing code. For future modules, libtbx.find_clutter will complain if that line is missing. A script libtbx.add_from_future_import_division is provided that can add that line correctly.

The rationale is that:
i. as you know, we want to use the so-called new division semantics in cctbx modules (i.e. 1/2 == 0.5 instead of 1/2 == 0 as per the old semantics for the record)
ii. the current solution passing -Qnew on the Python command line in the dispatchers libtbx.python, cctbx.python, etc is not always appropriate as there are people out there using cctbx as yet-another-library in a context where some code would not correctly run with the new division semantics, therefore barring the use of -Qnew

The only solution to reconcile i. and ii. is therefore to add that __future__ line in every single Python module committed to the cctbx.

Note however that the aforementioned dispatchers do still use -Qnew so that Phenix developers do not need to put that very line in their Phenix code.

Hopefully, we can make everybody reasonably happy with those compromises.

Best wishes,

Luc