Feature automated Morse restraints during bond creation

[akalpokas]

Changes proposed in this pull request:

This PR expands on the automated Morse restraint parametrisation to support alchemical contexts where a bond is alchemically being created, for example in a ring-forming case. Specifically it:

• It updates _restraints.py sire python API to support this, while specifically preventing the user from automatically parametrising a molecule containing multiple bond annihilations or creations simultaneously. We currently don't have a test system which requires such functionality, so therefore I haven't added support for it as of now.
• Adds an additional test for automatic parametrisation in tests/restraints/test_morse_potential_restraints.py for a propane_cyclopropane.bss system which requires an alchemical bond creation. (I skip the CI in commits to not trigger a sire build without the input file uploaded to test suite, however it should work when uploaded).
• Updates the Morse restraint tutorial and changelog to reflect the changes.

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• 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

Suggested reviewers:

@lohedges

[lohedges]

Thanks for this!