Hvelocity hvector limit support

This is a revision of my first stab at adding the limit method to HVelocity in previous pull request. 

I wasn't sure of the timeline or feasibility w/r/t HVector inheriting from PVector so my temporary solution in order to get limiting to work was to just add basic vector methods like mag/normalize straight to HVector.

Rather than use PVectors as properties of HVelocity, I instantiated a temporary HVector just to do the limit math.

There may be some value in vectors for getters and setters in HVelocity too - like .loc(HVector location), .velocity(HVector velocity) etc. I can do that in a separate pull request to keep things organized.