This branch adds an implementation for a data transfer of DOFVectors during calls to adapt.

Added:

• Data transfer implementation
• TreeContainer class supporting access to data via treepath argument (by @spraetor)
• TreeContainer helper class
• Unit test for data transfer using ProblemStat

Changed:

• Data transfer interface is now more general
• Tests using DOFVector<float> now use double. DOFVector<float> causes ambiguity in interpolation until dune-common MR 637 is available.

Requires:

• !81 (merged)
• !83 (merged)
• [https://gitlab.dune-project.org/core/dune-common/merge_requests/637]

WIP:

• Add a test for the data transfer
• Separate restrictLocal calls into pairs of father and child element and reduce the number of function arguments
• Replace the dummy bool argument of DataTransfer by something meaningful (a Grid type of sorts to allow special implementations)
• (optional) Change TreeContainer to allow mixed data types
• Cleanup: Comments, Add/Remove includes, feature checks
• Wait for required MRs to be finished