Extensions to interpolate() for DGFunction and DG1Function

When a DGFunction uses a standard nodal basis to represent the local polynomial on an element, we can implement interpolation in a straightforward manner. This MR has two components:

• DGFunction::interpolate() for the case of a constant now tests, whether basis_ is a nodal basis, i.e. of type DGBasisLinearLagrange_Example. If this is true, we set all DoFs to the prescribed constant value. If not, we abort.

• It brings an implementation of DG1Function::interpolate() for the case of an expression. Note that the evaluation of the expression already uses the micromesh functionality. A step in direction of resolving issue #293.