Use new heuristic planner

[Maumagnaguagno]

The new heuristic planner fixes a lot of small bugs, but uses certain different structures and Python 3.x, which may be a problem during merge.

Remember:

• sys.maxint is equivalent to float("inf")
• tuple goal_state is broken into positive_goals and negative_goals

[meneguzzi]

Did this bug disappear? I had forgotten about it.

[Maumagnaguagno]

I thought this was solved long ago, as people updating this repo are probably using Python 3.x and either the small bugs got fixed or are never reached.
Perhaps I was too optimistic, better leave this issue open.