[Moco] Switching a MocoStudy to implicit dynamics after constructing from MocoTrack causes error

[nickbianco]

Posted on the Moco forum.

If you construct a MocoTrack object, call "initialize()" to export the internal MocoStudy, and then switch the problem to use implicit mode, Moco issues the following error:

java.lang.RuntimeException: casVector should be 1-dimensional, but has size 0 x 0.
Thrown at MocoCasOCProblem.h:108 in convertToSimTKVector().

Rather than this cryptic error message, we should throw an exception that states that the problem is incompatible. We should also add a note to MocoTrack to tell users that they must update the guess if they change the problem to use implicit mode.