Fix AtomMapping::alignTo when mapping an ion to a molecule

[lohedges]

This PR fixes the AtomMapping::alignTo methods to handle the case where one molecule is a monatomic ion. This allows sire.morph.merge to handle the creation of alchemical ions by merging a water molecule with an ion. The existing alignTo methods fail since they attempt a RMSD alignment when there are too few degrees of freedom.

• 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]

Note that this is just assuming a single ion is mapped, which is the use case in question. I guess we could generalise this to fix when the AtomMapping contains multiple molecules that are mapped to ions.

[lohedges]

I just need to add a platform skipif decorator for Windows. I'll add this tomorrow.

[chryswoods]

Thanks - nice fix :-)

[lohedges]

Thanks. I'll merge this in so that we can work on the somd2 alchemical ion implementation next week.