Have thoroughly established that the problem is argper

In some cases, argper should be 2pi - argper

Plotting orbit tracks for argper and 2pi -argper shows that
if one is right, the other is wrong (and vice versa)
Seems to be fixed - using (n x e) .h appears to select correctly:

(n x e).h > 0.0 --> omega = 2pi - omega

Running stress tests to check