Add moving frame that slerps user-defined normals

[ecurtiss]

We already have the main infrastructure to support this, namely rotation-minimizing frames. It remains to create a spline that slerps the angles of the normals with respect to the RMFs.

[ecurtiss]

CatRom now supports number-type splines, so we can use CatRom internally to interpolate the roll angle. This is not necessarily the best decision, as we might want to impose different constraints on the roll spline (ex. minimize acceleration), but it is a convenient first choice.

Note that this PR depends on #26.