playground executable for experimenting with solvers and preconditioners

This MR adds an executable istl-playground that can be easily used to try out solvers or preconditioners. It reads a matrix and a rhs from MatrixMarket files or generates it if none is given and solves the systems with a solver from the matrix factory. This works also in parallel. The matrix can either be loaded on one rank and then be distributed on the ranks using ParMetis, or can be read from a distributed MatrixMarket file. As it is probably rarly used I added the EXCLUDE_FROM_ALL tag but added it as a test to avoid bit rotting.

I used it a lot in the last time. I hope that others could also benefit from it.

bin/CMakeLists.txt

+add_executable(istl-playground EXCLUDE_FROM_ALL istl-playground.cc)
+add_dune_all_flags(istl-playground)
+dune_symlink_to_source_files(FILES playground.ini)
+dune_add_test(TARGET istl-playground)