Fix annotate for chunked operations like in insulation

[Phlya]

Simple solution with .loc instead of .iloc - probaly a bit slower, perhaps some arithmetic can fix the .iloc to behave correctly

[Phlya]

I had to add the recently removed export of _IndexingMixin, since cooltools uses it...

[Phlya]

@nvictus would you be able to review? We should release a fix asap

[nvictus]

This needs a test case that triggers the error without the fix.

[nvictus]

I had to add the recently removed export of _IndexingMixin, since cooltools uses it...

We can restore it in a patch, but cooltools needs to vendor it in, because it is not part of cooler's public API and I'll remove it again in the next minor release.

[Phlya]

Thank you @nvictus!
For now I am keeping the _IndexingMixin, since without it cooltools just breaks completely... We can migrate it in the next cooltools release and then remove from cooler.
Added a test, it fails in the master branch but works in the version here in my local tests.

Of course numpy 2.0 breaks stuff, so the tests here fail...

[nvictus]

Great opportunity to throw in the numpy major version pin.

[Phlya]

Looks like disallowing numpy 2.0 makes the tests pass! Shall we merge and make a bugfix release?