[MRG] Fix RMSE Annotation

[kmilo9999]

On this PR we are enhancing the RSME annotation produced when comparing diploes. We stop overwriting annotations from previous runs. We also add a message in the gui console with the information from the last comparison. solves #710

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 92.62%. Comparing base (19aff25) to head (c37ae47). Report is 1 commits behind head on master.

:exclamation: Current head c37ae47 differs from pull request most recent head 7b4de47. Consider uploading reports for the commit 7b4de47 to get more accurate results

Files	Patch %	Lines
hnn_core/gui/_viz_manager.py	0.00%	7 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #741 +/- ## ========================================== - Coverage 92.73% 92.62% -0.12% ========================================== Files 27 27 Lines 4917 4923 +6 ========================================== Hits 4560 4560 - Misses 357 363 +6 ```

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[jasmainak]

perhaps @rythorpe @ntolley can help review ?

[gtdang]

This branch needs a rebase to clear up some of the diffs that there merged from #730.

[kmilo9999]

@gtdang Rebase done. This PR is ready to review

[jasmainak]

Looks good! Can you squash your commits and add an entry in whats_new.rst so we can give you credit when making the release?