Handle demo mesh location better

[Martin Sandve Alnæs]

For the demo problems to be importable from anywhere, they need to get the mesh in a more robust fashion than the current '../../data/...' path.

[Øyvind Evju]

I recently changed

../../../data/pipe_0.5k.xml.gz

to

path.join(path.dirname(path.realpath(__file__)),"../../../data/pipe_0.5k.xml.gz")

in poiseuille3d and womersley3d demos.

Do you want something better than that?

[Martin Sandve Alnæs]

It's not pretty but as long as it works its fine.