[WIP] Feature/functions

For 3.0 we discussed to build upon dune-functions.

This branch is the work-inprogress-approach to introduce the new dune-functions features in pdelab. I plan to include the following features:

• interpolate from a dune-functions or dune-pdelab function or any callable
• create a discrete function, which filfills the dune-functions interface:
  • create leaf functions DiscreteGridViewFunction
  • create trees of discrete functions
  • add extended interface for pdelab, including setTime:
    • concept check for setTime
    • type erasure class to the extended pdelab interface, including setTime
    • free standing setTime function

Things I don't plan for this brach, but which might be nice, or necessary, for the future:

• update GridFunctionSpace to adhere to the dune-functions interface. This a big thing! 😨
• cleanup the parameter interface to make use of the new functions features.

These features are large enough to have a feature branch on their own 😃