Add import mappings for entity alternative

[soininen]

Added import_mapping.EntityAlternativeActivityMapping that allows importing entity alternatives. To facilitate its use in Importer, parameter mappings now have AlternativeMapping as the root mapping replacing ParameterDefinitionMapping. This should not have other effects as the order is not important in this case.

Re spine-tools/Spine-Toolbox#2748

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

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

Project coverage is 78.34%. Comparing base (faa77ef) to head (0dcda6f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #425 +/- ## ========================================== + Coverage 78.29% 78.34% +0.04% ========================================== Files 72 72 Lines 9575 9595 +20 Branches 2044 2048 +4 ========================================== + Hits 7497 7517 +20 Misses 1738 1738 Partials 340 340 ```

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