Issue #1797 fix api generate index array

[JoerivanEngelen]

Fixes #1797

Description

This PR changes the following:

• Fix so that public API method GridData.generate_index_array returns the same as it did in iMOD Python 1.0, throw DeprecationWarning urging users to update to GridData.generate_isactive_svat_array.
• Further improve new name method to GridData.generate_isactive_svat_array and add it to public API docs
• Refactor methods in GridData somewhat, to reduce code duplication.
• Start renaming the unfortunately chosen variable name index to the more descriptive isactive, as the variable is in fact NOT an index. In a further PR we can rename this variable throughout the complete MetaSWAP module, which will affect a lot of files. I think that is worthy a separate PR.

Checklist

• Links to correct issue
• Update changelog, if changes affect users
• PR title starts with Issue #nr, e.g. Issue #737
• Unit tests were added
• If feature added: Added/extended example
• If feature added: Added feature to API documentation
• If pixi.lock was changed: Ran pixi run generate-sbom and committed changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud