Get information about binaries in PeTar

[rieder]

This adds a way of getting information about binary companions of particles in PeTar. cc: @lwang-astro @cournoyercloutierc

[rieder]

Not ready for merging before the following is fixed:

• [ ] don't write files to disk in addition to reporting via AMUSE
• [ ] ensure we consistently report the correct companion (e.g. even after deleting/adding particles)
• [ ] add test functions for this functionality

[rieder]

@cournoyercloutierc have you been able to try this?