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.

Various bugfixes on the interpreter.

Wrote most of the interpreter, need to find bugs now to get it to work.

A lot of work on World and the interpreter. ...

This is the applicative branch - I cut out the OptionCommand class, using
method calls to stored function names for options instead.