Add RigidBody::getMobilizedBody() convenience.

Fixes #207.
This line is giving me a heart attack (not your fault)! I hope someone knew what they were doing here since the original Rotation is column ordered while a C doubly-indexed array like `rDirCos` is row ordered. That means the matrix is effective transposed by this operation. No doubt it gets used in some transposed way later so it all works out in the end, but ... yuck.