CtoGParticleException added (presently unused)
moved colorbraiding.crossingsToGenerators to a separate file
Decode which particles cause coincident error. Resolves Issue #82

Both crossingsToGenerators.m and crossingstogenerators_helper.hpp now return
sorted indices (colors) of coincident particles in error message,
which is then decoded by colorbraiding.m to match the indices of "slices" of XY
that are the cause of the error.