Add submodule `parcels._core.sgrid` and tests #1912

Job	Run time
cache-pixi-lock / Generate output	1m 8s
Unit tests: ubuntu \| pixi run -e test-latest tests	3m 48s
Unit tests: ubuntu \| pixi run -e test-minimum tests	3m 30s
Unit tests: windows \| pixi run -e test-latest tests	8m 50s
Unit tests: ubuntu \| pixi run -e test-py311 tests	3m 36s
Unit tests: ubuntu \| pixi run -e test-py313 tests	3m 39s
Integration: ${{ matrix.os }} \| pixi run -e ${{ matrix.pixi-environment }} tests-notebooks	-1s
TypeChecking: pixi run typing	-1s
merge-test-artifacts	-1s
	24m 28s

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