Multiple planet system models (BT1P and BT2P)

[jorisverbiest]

Adding multiple planetary companions would be a nice (if niche) addition.

There is some functionality for this already in tempo2, particularly in the T2 binary model (key word "companion"), but it's not fully implemented there, either.
Presumably a nice first implementation would be to have a few companions in simple BT orbits, although in some cases perhaps the mutual interaction between the companions may need to be included, which would be far more complex.