Add submodule `parcels._core.sgrid` and tests #1913

Job	Run time
cache-pixi-lock / Generate output	13s
Unit tests: ubuntu \| pixi run -e test-py311 tests	3m 52s
Unit tests: ubuntu \| pixi run -e test-minimum tests	3m 42s
Unit tests: windows \| pixi run -e test-latest tests	8m 46s
Unit tests: ubuntu \| pixi run -e test-py313 tests	3m 49s
Unit tests: ubuntu \| pixi run -e test-latest tests	3m 50s
TypeChecking: pixi run typing	0s
Integration: ${{ matrix.os }} \| pixi run -e ${{ matrix.pixi-environment }} tests-notebooks	0s
merge-test-artifacts	1s
	24m 13s

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