Introduce parallel Core/DRU structures

[forman]

Restructured Gavicore and Wraptile around OGC API – Processes Part 1: Core and Part 2: Deploy, Replace, Undeploy.

Closes #168

Introduced parallel Core/DRU structures in various modules and mirrored it in the test modules. This is a breaking change: former module paths and package-level class exports are removed without compatibility aliases:

• Split Gavicore models and service interfaces into gavicore.models.core|dru and gavicore.service.core|dru modules, retaining shared error helpers.
• Split Wraptile routes into wraptile.routes.core|dru, with the application-package response class alongside DRU routes.
• Mirror the structure in tests and update imports, generators, notebooks, docs, and module docstrings.
• Preserve Core and conditional DRU route registration.

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[forman]

FYI @Florian-Katerndahl