Fix issue #189

[lohedges]

This PR closes #189 by adding the missing TriclinicBox::maximumCutoff method. This uses the approach from OpenMM, i.e. we use the theoretical maximum of half the norm of the smallest vector when the box isn't in reduced form, but limit it to the minimum diagonal element of the box vector matrix for efficiency when it is in reduced form. (See the discussion here and the linked PR here.)

• 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]

Just to clarify, I decided to use the approach from OpenMM since it means that a reduced box generated by sire or BioSimSpace will set the correct cutoff when used to create an OpenMM context, which is probably our most typical use case. It should also be fine for other engines too. (It is just on the conservative side.)