critical potential check should be done at closest approach

[kecnry]

When using nbody dynamics, we need to check for roche overflow at the time at which the two bodies are closest to each other (within all datasets).

This will be difficult - as it requires running nbody on the orbits in real time (ie. before run_compute). Alternatively, we could leave the system checks as is but then run the more robust check in the beginning of run_compute before entering the per-time loop. Or possibly having a "robust" mode to run_checks that can also be called manually.