a) some intrinsic flexibility associated with the first crossover
b) the spring for the twist of the helix
c) intrinsic flexibility associated with the second crossover

1) Proper accounting for a single bond representing a double crossover
2) Parallel-helix dihedral angle spring constant is now computed from three "springs" placed in series that represent:
   a) the flexibility of the bond on one side of the crossover
   b) the intrinsic flexibility of the crossover
   c) the flexibility of the bond on the other side of the crossover

Fixed bug introduced in commit 47b578f2 where ArbdModel.__init__ arguments were incorrectly provided by SegmentModel.__init__

Prepared code structure in potential section so that dsDNA and ssDNA are treated using separate defs

Fixed issue affecting systems with long polymers where update_splines previously used float16 to obtain unique beads; now rounding to nearest 1/10th nt

Improved orientation potentials at junctions when local_twist is enabled; now dihedrals are more local and work in more situations

Made it so a bead can be located at multiple distant locations within a segment (e.g. for looped constructs)

1) Fixed lattice_type setting for cadnano structures, affecting ENM
2) Raise exception for circular segments (sites that loop such as RNA stem loops remain problematic)
3) Made translate and rotate functions return if nothing is selected by filter