Implement fieldset._load_timesteps() #1145

Job	Run time
py3.11 \| ubuntu \| unit tests	6h 0m 15s
py3.12 \| ubuntu \| unit tests	6h 0m 24s
py${{ matrix.python-version }} \| ${{ matrix.os }} \| integration tests	-1s
mypy	-1s
merge-test-artifacts	0s
	12h 0m 37s

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