add Marching Tetrahedrons code
change MarchingCubes.march() to take a VoxelData rather than a SampledField
MarchingCubes.java: snap points to grid if they are close; this eliminates many mesh irregularities
comment
adjust MarchingCubes to use sampling fields where positive values are the interior
refactor MarchingCubes to use only a single static method (never instantiated)
reformat MarchingCubes, some refactoring
add first draft of Marching Cubes codefg