Re-enabling Py2 compilation

Relates Fix to #144 (closed).

• One Py3-required LOG_TRACE (introduced in 54439906, for the record) optionally removed
• Making Py3 mandatory for MPI compilation.

Note that CMakeLists.txt format could be improved but I did not introduce formatting (indentation) changes, for now.

Startup may crash because of a fancy rightwards arrow l. 78 of generated install/lib/**/py/yade/__init__.py:

## ENABLE_LOGGER ## this was in git revision 014b11496 YADE_LOG4CXX → YADE_BOOST_LOG ##

Manually removing this arrow would solve the startup problem.. Would this by chance ring a bell to anyone ? (as for a correct solution)