Add "remove low quality simplex" functions. #18
Description
Try to improve the quality of the simplices by safe operations (i.e. where the interface is ensured not to be changed). This includes:
- Remove "cap" tetrahedra.
- Remove "needle" tetrahedra.
- Remove "wedge" tetrahedra.
- Remove "sliver" tetrahedra.
- Remove "cap" faces.
- Remove "needle" faces.
- Remove short edges.