[linearelliptic.discretizations.base] keep a copy of the boundary cfg
[linearelliptic.discretizations...] let the la_backend default to istl
[linearelliptic.discretizations...] add access to sparsity pattern
[linearelliptic.discretizations.swipdg] use correct default_beta
[linearelliptic.problems.zero-boundary] added first draft to playground
[examples.linearelliptic] undef HAVE_DUNE_GRID_MULTISCALE
[linearelliptic.discreteproblem] added first draft of DiscreteBlockProblem
[linearelliptic.discretizations...] add BlockSWIPDG as a friend