Adding and updating water models
This release adds opc-1.0.1.offxml, standardizing the use of the string "kilocalorie_per_mole". It also adds tip3p-fb-1.1.0.offxml, which fixes a unit error in the geometric constraints and adds additional ion parameters. Finally, it adds opc3-1.0.0.offxml and tip4p_fb-1.0.0.offxml.
See docs/water-models.md for more information, and the scripts/ directory for the code to regenerate the force field files.