Add submodule `parcels._core.sgrid` and tests #1966

Job	Run time
cache-pixi-lock / Generate output	12s
Unit tests: ubuntu \| pixi run -e test-minimum tests	44s
Unit tests: ubuntu \| pixi run -e test-latest tests	44s
Unit tests: windows \| pixi run -e test-latest tests	33s
Unit tests: ubuntu \| pixi run -e test-py311 tests	46s
Unit tests: ubuntu \| pixi run -e test-py313 tests	44s
TypeChecking: pixi run typing	0s
Integration: ${{ matrix.os }} \| pixi run -e ${{ matrix.pixi-environment }} tests-notebooks	0s
merge-test-artifacts	1s
	3m 44s

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