Merge branch 'feature/fix-call-MPIfinalize-onlyif-MPIInit-called' into 'master'

MPIHelper should only finalize MPI if it called init

MPIHelper only calls MPI if it was not already called before MPIHelper is constructed. Finalize is however always called on destruction leading to problems with other packages calling init/finalize before/after MPIHelper is setup. Added a bool storing the information if MPIInit was called by the MPIHelper so that finalize is only called in that case.

See merge request !58 (merged)