fix for the issue of merging of nested rows

[TermeHansen]

Issue number of the reported bug or feature request: #459

Describe your changes
Fix for the issue when merging row headers in nested indexes

Testing performed
Using the case from https://github.com/bloomberg/ipydatagrid/blob/0354a4d66c5c1298895026cec7d99bdb360914c1/examples/Nested%20Hierarchies.ipynb#L33

but changing top and bottom row and adding an extra element:

row_bottom_level = ["Sector 1", "Sector 1", "Sector 2", "Sector 2", "Sector 2"]
row_top_level = ["Security A", "Security B", "Security C", "Security C", "Security D"]

Current code gives:

Fixed code gives:

[martinRenou]

Thanks a lot for this!

Would you be able to add a test notebook under ui-tests-ipw8/tests/notebooks to test this?

Also you need to signoff your commits. You can signoff using git commit --signoff, this also work in combination with --amend to modify your existing commits.

[martinRenou]

Thanks @TermeHansen ! If you want I can push another commit to your branch to get the CI green

[TermeHansen]

@martinRenou sure, whatever gets this in 😄

[martinRenou]

I'll just get your visual regression test to pass, won't touch the rest of your code, thanks!