[dune-pdelab] Time dependent function types

[Christoph Grüninger]

Dear Dune folks,
by writing a parabolic solver I have not found time dependent function types. For elliptic problems and problems with constant boundary functions this was sufficient but we'll need them for most hyperbolic and parabolic problems.

In my case a time dependent analytic solution is given but I can hard code the evaluated point in time. Nevertheless the topic must be discussed and a proper solution must be found and implemented.

Regards,
Christoph