The general polygon version is not implemented yet because I need to
change the way it is interpolated.
To get wedge to work correctly, I had to change the interpolation
slightly there. Previously the interpolation had a singularity at
the tip.
The functions for doing interpolations and derivatives were called
CellInterpolate and CellDerivative, but the file names were
Interpolate.h and Derivative. Now the files are CellInterpolate.h and
CellDerivative.h so they are more consistent and a bit easier to find.
The interpolate and derivative functions has the shape tag at end of the
function arguments (just before the "worklet" parameter, which is just
the error handling mechanism). However, the parametric coordinate
functions had the shape tag at the beginning. Moved the shape tag to the
end to be more consistent within these functions and also in other uses
throughtout VTK-m.
