GitLab

* copy opc ions and opc*standard.xml from https://github.com/openmm/openmmforcefields/commit/b12c2e871b60275a10b3f8f56dfc2f6d5447591f

* auto-merged files

* move charges to residue definition

* remove unmerged files

* add docs

* rename U to U4+

* rename Ag+, Tl+, Gd3+ and I- residue names

* fix O-H bond length typo

* add opc3 constraints test