Port RF torsion fix to PME code

[lohedges]

I hadn't realised that the PME implementation existed in a completely separate class, so the torsion fix is missing there.

• 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 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]

Suggested reviewers:

@chryswoods

[lohedges]

I've merged #226 into this branch so both PRs can be merged in one go.

[lohedges]

This has now been updated with the latest commit from #226, i.e. the update to the equation used for the flat-bottomed distance restraint.