fix compile with dune 2.3
quell warnings related to the mpi helper with dune 2.3
quell warning related to constructor argument changes in dune 2.3
fixed: don't build known_answer_test with dune >= 2.3

uses removed features
changed: quell warnings about unused local typedefs
fixed: quell deprecation warnings when compiling with dune >= 2.3
Merge pull request #85 from akva2/fix_dune_2.3_issues

Fix dune 2.3 issues