Hacs64 is broken

[LourensVeen]

Describe the bug
Hacs64 compiles, but its tests crash when you try to run them. It seems the tests were disabled in 2012, it's unclear if they ever passed. This also has OpenMP support, but it wasn't enabled in the pip package at any rate, and it probably also doesn't work.

We need to decide what to do with this. It does have 6th and 8th order integrators, which is potentially interesting.