Enable pthread in docker containers to obtain full vader interprocess communications performance

See merge request !27

Add a python 3.9 based docker image with an up to date MPI (not used for CI).

See merge request !26

Resolve "Add python3.x support"

Closes #46

See merge request !24

Mpi operators

See merge request !25

Fix hdf writer in append mode (negative seek in text file is allowed only in binary mode in python3)

Fix ci cleanup in docker image (pip uninstall not works without pythonpath export and do not remove INSTALLDIR but INSTALLDIR/lib/site-package/*). Various fixes

Merge branch '46-add-python3-x-support' into 29-hysop-with-tasks