Let LocalDensity depend on the integration domain

Rather than just on the type for points in the domain.
The point type is not enough: For example, if the density involves
a coefficient function given as a GridView function, then the
density has to be able to bind the local coefficient function
to the correct element, for which it has to know the element type.

Integration domains can be elements (i.e., Codim<0>::Entity)
or Intersections.

The actual bind method will follow in a separate commit.