Release v2.3.1

• bugfix: GraftList.assign_residues() now removes base-structure residues downstream of the outermost receiver (and their links) before applying graft external links; previously, if the base structure carried any sugars beyond the outermost receiver resid, psfgen would apply both the existing glycosidic patch and the new graft external link to the same acceptor oxygen, producing a doubly-bonded OC301 and a CC3162 OC301 CC3162 CC3161 dihedral that NAMD cannot parameterize