Write documentation for demos

[Martin Sandve Alnæs]

Two parts: a module or class docstring describing the problem being solved, and maybe function docstrings describing how the problem interface is used.

It's ok if some demos are left undocumented for now. See also issue on restructuring demos.

Documentation should follow the FEniCS documentation styleguides: http://fenicsproject.org/contributing/styleguide_doc.html#styleguide-documentation

[Øyvind Evju]

I've added separate documentation.rst files in the same folder as the demos for FlowAroundCylinder, Womersley3D, Restart and Replay.

Should probably add a demo of more advanced fields as well, but then I think it's sufficient.