Adapt to COSY markers.

Marker assignments for the maniplandum and the target frame in a test
at COSY.
Work around for co-planar markers.

If the maniplandum markers that are visible are close to co-planar, the
best-fit transformation is ill-conditioned. Here we try to detect when
that happens and then re-compute the orientation, and then position,
based on only 3 of the markers.