- the blockMesh interface is splineEdge.H, selectable as "spline"

The first tests look fine - it works as expected for the case with
buggy polySpline reported on the forum. Should of course do some more
extensive testing.

The advantages compared to the current B-Spline implementation:

- Doesn't need a matrix solver.
- The coding resembles something that can be found in the literature.
- In contrast to the B-Spline implementation, it is fairly clear what
  is actually going on. I don't even know if the B-Spline are actually
  B-Spline, Beta-Splines or something else.
- Catmull-Rom splines seem to be what all the graphics people have as
  their stable workhorse.

We now have 3 different names for splines in blockMesh:
- "spline" - *new* Catmull-Rom for arbitrary segments.
- "simpleSpline" - B-Spline for a single segment
- "polySpline" - B-Spline for a multiple segments

Assuming the Catmull-Rom splines continue to behave nicely, there is
no reason to keep the other (broken) B-Splines. This would help clean
up the blockMesh interface too.

Placed the older ones under legacy/ for easier identification in the
future.

TODO:
- currently no handling of non-zero end tangents
- could be extended to handle closed loops, which might be useful
  for feature edges from CAD (eg, for the cvm mesher)