BUG: fix load_uniform_grid with cell_widths and multiple fields #8441

Job	Run time
unit tests: py3.13 on macos-latest (pytest)	13m 46s
answer tests: py3.10 on ubuntu-latest (nose)	14m 12s
unit tests: py3.10.3 on ubuntu-20.04 (pytest)	10m 49s
unit tests: py3.10 on ubuntu-latest (nose)	13m 44s
unit tests: py3.13 on ubuntu-latest (pytest)	17m 59s
unit tests: py3.x on ubuntu-latest (pytest)	10m 4s
unit tests: py3.13 on windows-latest (pytest)	17m 37s
Image tests	6m 1s
	1h 44m 12s

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