test: regression tests for the pandas-2 fixes in #1325

[EnvDroneSense]

Changes proposed in this PR:

• Add regression tests for the pandas 2.0 fixes merged in minimal fixes #1325, one per affected entry point: LitPop.from_shape_and_countries (GeoSeries shape, and overlapping shapes), impact_data.hit_country_per_hazard, calibration_opt.calib_all and calibration_opt.calib_instance (multi-row df_out with yearly_impact=True).
• Add the CHANGELOG entry for minimal fixes #1325.

The source changes originally in this PR were dropped in favour of #1325.

This PR fixes #1319

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
• CLIMADA Reviewer Checklist passed
• Tests passing
• No new linter issues

[emanuel-schmid]

Thanks for pointing this out, the suggested fixes and writing tests.
I've made some remarks. You can address them as you wish.

In any case, I'm going to just apply minimal fixes without much ado soon. 🤷 There bugs and ought to be removed.

[EnvDroneSense]

@emanuel-schmid thanks for the review (sorry for the delay, the semester has just started, so it was quite busy). I rebased onto your fixes on develop and reworked the tests as suggested:

• calibration tests merged into TestCalib, with plain docstrings; the multi-param test is now test_calib_all
• new test_overlapping_shapes for LitPop.from_shape_and_countries
• CHANGELOG entry covering minimal fixes #1325

So the PR is now is tests + changelog only.