Add support for ff19SB force field

[lohedges]

This PR builds off the new Sire CMAP feature to add support for the ff19SB force field. I've added tests to make sure that it can be used to parameterise a protein and that the resulting molecule contains a cmap property.

Supersedes and closes #398.

• I confirm that I have merged the latest version of devel into this branch before issuing this pull request (e.g. by running git pull origin devel): [y]
• I confirm that I have added a test for any new functionality in this pull request: [y]
• I confirm that I have added documentation (e.g. a new tutorial page or detailed guide) for any new functionality in this pull request: [y]
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[lohedges]

While parameterisation works, we need to think about water models to support ff19SB, e.g. adding support for OPC. This will need updates to the gmx solvate wrapper, as well as additional reference topologies in Sire for interconversion between AMBER and GROMACS representations.

[lohedges]

I've now added support for the OPC water model, which requires the feature_ff19sb branch of Sire. I'll use this for some test simulations to make sure things are okay.