Implement cell_avg(function) and facet_avg(function)

[Martin Sandve Alnæs]

I'm implementing these operators in UFL and FFC:

cell_avg(function) # Take the average value of function over the current cell facet_avg(function) # Take the average value of function over the current facet

where function is either an Argument or a Coefficient (not a general expression).

The FFC implementation will take the average of the basis functions by numerical integration during code generation. Thus no integration loop needs to be generated and the implementation is fairly straightforward. This is basically already in place locally, just needs some testing.