Now clustering nucleotide types rather than rounding; gives more accurate number of nucleotides in a double helix

Now initializing quaternion_spline_params attribute in Segment init to None; prevents errors when mapping atomic models of ssDNA

Improved scripts for building cadnano structures; seems it will be neccesary to make contour go from nt[0]-0.5 to nt[-1]+0.5

Changed how crossover dihedrals are written when local_twist is False; fixed routines that walk through helices