Fix ROI signal axes Merlin. Added tests

[kritvei]

Description of the change

A few sentences and/or a bulleted list to describe and motivate the change:

• Fixed bug in ROI for Merlin signal axes

Progress of the PR

• [x] Change implemented (can be split into several points),
• [ ] update docstring (if appropriate),
• [ ] update user guide (if appropriate),
• [ ] add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• [ ] Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:rosettasciio build of this PR (link in github checks)
• [x] add tests,
• [x] ready for review.

Note that this example can be useful to update the user guide.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.52%. Comparing base (37ea44f) to head (53e2617).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #289 +/- ## ======================================= Coverage 86.52% 86.52% ======================================= Files 83 83 Lines 10676 10676 Branches 2332 2332 ======================================= Hits 9237 9237 Misses 922 922 Partials 517 517 ```

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[ericpre]

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