This is a simple simulation in Python of a network of pipes containing gas, with plungers
to push it around.

Requires: Python, PyGame

The code is rather simple, split into the following classes:

Tile

A Tile is a square on the screen. It can either be "solid" (ie. a wall or a plunger) or not,
ie. an empty space. Tiles form the heart of the computation. Tiles should normally be created
automatically by Levels.

Level

A Level is a collection of Tiles, where each one knows of its neighbours. The best way to make
a Level is to use the static make_level factory method. It takes a string where 0 is empty and
# is a wall.

Plunger

A Plunger is a solid block which sits on a Tile, blocking gas from passing. Plungers can be
moved, for example this can be tied to keyboard input.

PassivePlunger

A PassivePlunger is a solid block which sits on a Tile and moves along pressure gradients.

To use, click on black squares to add gas to the system. Use the arrow keys to move the top
left plunger (yellow block) around, compressing and expanding the gas, and moving the other
plungers in turn.