Add support for computing trajectory RMSDs using Sire

[lohedges]

This PR exposes the new Sire trajectory alignment and RMSD functionality, making the Sire trajectory backend fully compatible with the existing MDAnalysis and MDTraj interfaces.

This PR requires OpenBioSim/sire#97. I'll cancel the CI and re-trigger once that PR has been merged.

• 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] (Already documented via existing API and website docs.)
• I confirm that I have permission to release this code under the GPL3 license: [y]

Suggested reviewers:

@chryswoods

[chryswoods]

Looks great - I've just merged feature_align into devel. It will take some time to build...