Split flying edges and marching cells into separate filters

The Contour filter is bulky and slow to compile because it actually contains two separate contouring algorithms. If the input has a CellSetStructured<3> cell set and an ArrayHandleUniformPointCoordinates for point coordinates, then it uses flying edges. Otherwise, it uses marching cells.

These two algorithms could be separated into two separate filters (i.e., ContourFlyingEdges and ContourMarchingCells). The main advantage of doing this is to split up the code to compile more efficiently into two translation units. This helps ease the burden on some device compilers and makes parallel builds more efficient.

After the split, there should still be a Contour filter. This filter would simply direct its input to either ContourFlyingEdges or ContourMarchingCubes depending on the input data type. The MeshQuality filter does something similar.