Separate fluid approximation from obstacle logic

The obstacle is now pure mesh geometry and does no more concern itself with the
fluid in its cells.
Change assertion to make it work with Python 2.6

We should at some point probably wrap assert_greater and assert_less, but here
it does not really make a functional difference, so I chose the easy way.
Merge pull request #53 from aepsil0n/refactor/obstacle

Separate fluid approximation from obstacle logic