Physics development progressing well

Written for PyIgnition by David Barker on 2010-06-19

Two of the three obstacle types initially planned for the next release have already been fully implemented, and are working rather nicely. The last planned one was a line segment, although that may be left until later as it will be incredibly difficult to implement it with the current system. In its place, however, I'm planning to add an infinite line (which particles can only be on one side of) - this will at least provide the boundary-defining functionality promised by the line segment, and will of course make it easy to create sloped areas and suchlike. The line segment is still planned, but for the moment will have to wait until a later release.