Numba-mpi

- added first version of `allreduce` function (only the SUM operation supported, no complex numbers yet) - thanks david-zwicker!
- refactored creation of test data in the test cases - thanks david-zwicker!

- pinning pip-publish action to `pypa/gh-action-pypi-publishunstable/v1` (was `master` which is now deprecated)

- fixed sending FORTRAN-ordered arrays (thanks david-zwicker!)

- support for other-than-double data types (thanks david-zwicker!)

- improved handling of 0-d arrays (thanks david-zwicker!)

- allow sending and receiving arrays that are not contiguous (thanks to david-zwicker!)