branched polymer axes

[skfegan]

When creating the residue level rotational axes and the united atom translational axes (should be the same), the theory is to use the bonds to neighbouring residues ("tether points") to define the axes. Currently, this works for linear polymers. For branched polymers, we would need to refine the searching for bonds between residues to capture links between residues that are not next to each other in the topology list.