Merge branch 'feature/FS1705-eigenvalue-solvers'

cmake/modules/AddARPACKPPFlags.cmake

cmake/modules/DuneIstlMacros.cmake

cmake/modules/FindARPACK.cmake

cmake/modules/FindARPACKPP.cmake

config.h.cmake

configure.ac

dune/istl/CMakeLists.txt

dune/istl/eigenvalue/CMakeLists.txt

dune/istl/eigenvalue/Makefile.am

dune/istl/eigenvalue/arpackpp.hh

dune/istl/eigenvalue/poweriteration.hh

dune/istl/eigenvalue/test/CMakeLists.txt

dune/istl/eigenvalue/test/cond2test.cc

dune/istl/eigenvalue/test/matrixinfo.hh