Feature cell

[lohedges]

This PR adds support for passing box cell vectors through to EMLE, allowing calculation of terms that require box information during NPT dynamics. The change is backwards compatible, so existing EMLE code is unaffected.

The PR also adds the --install-metadata option to the setup.py script, allowing a non conda-build install of Sire to install metadata such that conda is aware of the installed package, i.e. it won't reinstall Sire if you pull in other conda packages that depend on Sire.

• 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]
• I confirm that I have added a changelog entry to the changelog (we will add a link to this PR as part of the review): [y]
• I confirm that I have permission to release this code under the GPL3 license: [y]