MetOffice/lfric_core: vn3.0

LFRic Core vn3.0 Release Notes Full Changelog: https://github.com/MetOffice/lfric_core/compare/core2.2...vn3.0 This is the first GitHub release of LFRic Core. Please see the refreshed Working Practices for details on how to migrate branches and work with GitHub. Key Changes Additional infrastructure has been added to support flexible checkpointing in LFRic Apps. This includes, a key_value_collection_iterator class, string splitting functionality, and some refactoring of the lfric_xios component to allow multiple checkpoint files to be written (ticket:4020). Mesh generators now have OpenMP support for generating partitioned meshes, enabling generation of meshes in a reasonable time when running at scale (for example, running a 10km global model on several thousand ranks can now be done in a few minutes on 32 cores). Sphinx based documentation hosted on GitHub Linked Codebases This release is part of the 2025.12.1 Simulation Systems Release.