Fixed math error in rendezvous autopilot match velocities step.

In some cases it would take the square root of a negative number and then try to create a maneuver node at UT = NaN, causing all sorts of problems.
Changed sepratron recognition logic again.

Now any engine that activates in the same stage in which it decouples is called a sepratron.
Changed menu position
Merge branch 'dev' of https://github.com/MuMech/MechJeb2 into dev