Constraints (update)

Three commits:

1. FIX the feval and ceval counting for all cstrs algorithms. These were growing exponentially over consecutive evolutions. Fevals and cevals were effectively counted multiple times, because new/cloned populations with old problems keep the old feval and ceval counters and do not reset them. The initial values thus have to be substracted.

2. REVERT for testing purposes crowding distance was exposed to PyGMO, this is undesired, so it is reverted.

3. REVERT main.cpp was altered with code that required nondefault options (PyKEP) and will thus break for most people. Since build_main is on by default the file should only contain simple code that will run for every configuration.