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.

Removed the direct solver specialization ofr double types in favour of the separate initialization in the creators

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