Make sure that reference frames in multi-star systems don't overlap 

This hopefully fixes #2344.

While ensuring that nothing could overlap in the worst case, I noticed that the minimum distance of binaries was set too low in StarSystem::MakeBinaryPair. Instead of checking that the periastron is beyond surface, the code checked only the semi major axis, which is larger in general. This is fixed in the first commit. It also ensures that binary stars are separated enough to fit planet systems and reference frames.

The second commit finally ensures that the reference frame of one binary does not intrude into the other's frame. If we wanted to increase to more than 0.11 * orbMin, we would have to increase the minimum binary separation beyond 1.25 radii.