BMesh: add BM_vert_splice_check_double

checks if splicing verts creates duplicate edges
Cleanup: assert / warn
initial intersect code
initial isect
intersection script
test vert intersection
check for intersections where no new verts are added first
minor fixes
WIP
add vert-edge check
Print info
De-duplicate - dont add same verts to edge multiple times
Dissolve
Dissolve based on tags
improve intersection dissolve
Dissolve now works
WIP
WIP
WIP
WIP