Updated uniform spacing modifiers

[marip8]

Motivation

The existing uniform spline tool path modifier often causes crashes and has a lot of unecessary overlap with the uniform linear tool path modifier, which is capable of doing spline interpolation using vtkTransformInterpolator. Looking into this more deeply, it appears that the full spline interpolation method of vtkTransformInterpolator does not create valid pose rotations. You can however set the position interpolation to be splined and the orientation interpolation to be linear which fixes the issue somewhat, but the resulting rotations (specifically the x-axes) visibly do not track the spline path very well.

The vtkTransformInterpolator and vtkSpline classes do not give access to the spline derivatives, so it's not really possible to correct the orientations after the fact.

Also, it appears that the vtkParametricSpline::ParameterizeByLength option evaluates length as the linear distance between knot points, not the actual length of the spline between them, leading to some inaccuracy.

Solution

Using the spline code developed in #360, it was relatively straightforward to replace the vtkTransformInterpolator with an Eigen-based B-spline to do uniform re-sampling of a tool path. A B-spline of degree 1 is also equivalent to linear interpolation, so this method is capable of doing both linear and polynomial spline interpolation. This method also calculates the spline arc distance using numerical integration of the spline first derivative with Simpson's rule which results in more accurate waypoint spacing.