Expose Sire "make whole" functionality

[lohedges]

This PR exposes the new make whole functionality in Sire. Users can now specify whether to unwrap molecules on read using the make_whole keyword argument. I've also exposed the system.make_whole() method, which calls the underlying Sire function to unwrap all of the molecules in the system.

I've also taken the opportunity to fix a few issues related to the BioSimSpaceTutorials repository. These involve minor updates to call .value() on objects that now have length units.

Closes OpenBioSim/BioSimSpaceTutorials#4
Closes OpenBioSim/BioSimSpaceTutorials#6

• 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: [n] (Tested via Sire)
• 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] (API docs)
• I confirm that I have permission to release this code under the GPL3 license: [y]

Suggested reviewers:

@chryswoods

[chryswoods]

This all looks good. It's nice to see the functionality picked up so quickly, and that it is able to be integrated easily.