Add submodule `parcels._core.sgrid` and tests #1936

Job	Run time
cache-pixi-lock / Generate output	17s
Unit tests: ubuntu \| pixi run -e test-latest tests	3m 40s
Unit tests: ubuntu \| pixi run -e test-py311 tests	3m 37s
Unit tests: windows \| pixi run -e test-latest tests	8m 3s
Unit tests: ubuntu \| pixi run -e test-py313 tests	3m 40s
Unit tests: ubuntu \| pixi run -e test-minimum tests	3m 40s
TypeChecking: pixi run typing	0s
Integration: ${{ matrix.os }} \| pixi run -e ${{ matrix.pixi-environment }} tests-notebooks	0s
merge-test-artifacts	0s
	22m 57s

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