Add documentation to boundary conditions
Correct DirichletBC::fillBC to use column basis for solution

Added init-finalize also for vectors.
Require gather/scatter operations instead of direct vector/matrix access.
Rewritten the interpolation.
Added ParallelIndexSet and DOFMapping to be used also in other backends.

make amdis compatible with dune 2.7, i.e. rewrite filleBoundaryCondition by passing a treepath additionally