Akin's screw notation seems backwards from most conventions

[mxgrey]

Akin is using (linear, angular) for screws, but it looks like most conventions (e.g. Featherstone and Plucker) use (angular, linear). This should be investigated further and corrected to match the most common or most sensible convention.

[mxgrey]

It seems that Jacobians usually use a (linear, angular) convention, which was the original motivation for having Screw represent (linear, angular). Consistency within the library is probably more important than matching outside conventions, so it seems likely that I will stick to (linear, angular) for all screws. Still, this all bears further consideration.