Minor bug fixes, started on player.py

Merged branches 'applicative' and 'applicative-remodulate'. ...

The basic structure of the ATL interpreter now stands and (sort of)
works, still some bugs though. And obviously plenty of features left
to add (currently implemented: define-place, description, neighbours).

Ready to merge back into master.

Reorganized the modules, move DefineCommand into new module define.py ...

This avoid circular dependencies.