Added support for having multiple intrahelical connections from a single "location" in a segment; allows two ssDNA strands to emerge from the ends of a dsDNA segment

Code now always places a ssDNA bead when an ssDNA segment is between two dsDNA segments.

Code now more robustly places a bead and intrahelical junctions

Updated algorithm that fits splines into beads to extend farther into intrahelically connected segments, and to use linear interpolation for quaternions, resulting in better all-atom models

Updated bead model potentials to work with antiparallel DNA; also made terminal_crossovers bond shorter at 1.2 nm (vs 1.85)

Added type_ argument to SingleStrandedSegment.add_crossover() for compatibility with DoubleStrandedSegment

Updated imports to be compatible with new package structure; updated paths to resources to use new get_resource_path() function

Made bond potentials longer ranged (120 vs 50 nm), and removed unneeded 'continue' statement that was attempting to prevent double-counting when placing initial beads