Finalize fvm_multicell

* Fix bug in 'close' event delivery for multicell.
* Fix displacement scaling bug in gather_all_with_partition.
* Command line parameter io, reporting for new miniapp options.
* Only master rank performs command line parameter writing.
* Add test for mpi::gather_all and mpi::gather_all_with_partition.
* Extend utility of `util::assign`, `util::append`.