blango - stl file splitter
- find middle of x extents
- collect left only triangles
- collect right only triangles
- collect intersecting trianges
- split intersecting triangles into 4 triangles (3 on large side one on small)
- save off vertices of splitting lines
- move new triangles into left and right collections
- create surface triangles for cut
- copy surface triangles into both left and right collections with opposite face vectors.
- write left and right files by extending filename with _left and _right