Feature repulsive morse

[akalpokas]

Changes proposed in this pull request:

This PR introduces a repulsion term to the Morse potential. See the internal discussion for details. I haven't updated the change log entry yet as I am submitting this PR from my phone (can do that later in the day)

• 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: not relevant
• I confirm that I have added documentation (e.g. a new tutorial page or detailed guide) for any new functionality in this pull request: not relevant
• 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): n
• I confirm that I have permission to release this code under the GPL3 license: y

[lohedges]

Thanks. No worries about the CHANGELOG. Since DMR was only added as part of this development cycle this addition can be absorbed into the original entry.

[lohedges]

Thanks for adding this, and for the detailed investigation.