Test merge functionality

[sdruskat]

Similar to #351, but going beyond core model functionality to look at merge functionality.

TODOs

1. Add content in Ha Ha Hack-A-Thon! 🐱‍💻 #288 that relate to merge functionality to https://github.com/softwarepub/hermes/tree/refactor/data-model (work with @led02 to make sure everything is in there).
2. Collect cases that should be covered by merge functionality, comprehensively.
   • For each case, create a new sub-issue of Test and refactor base model #350 that describes the case, input, expected output, etc.
3. For each issue, create a test that covers this case.
   • The creation of the test case is also an implicit test of the merge API!
     Whenever you encounter something in the merge API that deviates from your expectations, record this in a new sub-issue to this issue and link it to Refactor core model #352.