When implementing a frame based on the PV of another object (E.g. Sun
pointing, LOF) one always needs the angular velocity. I propose adding a
method called getSpin() or getAngularVelocity() to PVCoordinates to
compute this vector. I've noticed myself repeating the unintuitive
phrase
"pv.getMomentum().scalarMultiply(1.0/pv.getPosition().getNormSq())" and
would like to move it to its own method. This would complement the
existing method, getMomentum(), which computes the specific angular
momentum. I'll write the javadoc and test cases. WDYT?