Test omuse interface with Python 3 and on different systems

[fjansson]

• test OMUSE interface with Python 3, mention supported Python versions in metadata table of the paper.
• test installation on Ubuntu

Tested so far

• Python2, Fedora.
  • Dales interface should work with latest amuse from git, amuse-framework-12.0.3 on PyPi is missing an improvement in netcdf detection.
  • building POP needs work-around for nf-config not reporting module path (fedora packaging bug). Bug reports: Fedora26, Fedora30
• Python2, CentOS7 (with Singularity)
• Python2, Arch Linux