Feature/cellwise constant

Make it possible to mark Expressions as cellwise const and put evaluation in pdelab outside the quadrature loop. Closes issue #27.

The implementation is kind of sketchy but I wanted to go to weekend. Some notes/TODO:

- I had to add a statement to the ufl file. For cellwise const expressions a dg0 dummy space is used (-> cell wise const), else we use a dg1 space (in execution.py).
- I now use 'default_declaration' from loopy.py for defining parameter objects.

See merge request !14