Add falcon module to AMUSE

[rieder]

@eugvas has written an interface for the fast-multipole gravity solver "falcON" (W.Dehnen, ApJL, 536, 9, 2000) from NEMO: https://github.com/GalacticDynamics-Oxford/pyfalcon Let's see if we can include this in AMUSE / what is needed to do this.

[rieder]

It doesn't yet have unittests - these would need to be added before we could merge this in AMUSE

[eugvas]

here is my first attempt for a unit test: test_pyfalcon.py.txt

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[jnibauer]

It would be great if this could be opened back up again! I think having a falcon implementation in AMUSE would be very helpful for many.

[rieder]

Looks to be in good shape. I'll test when I can and make a PR.