ENH: add a convenience `nfields` property to `GasDataSet` #1247

Job	Run time
tests (ubuntu-latest, 3.12)	1m 0s
tests (macos-latest, 3.12)	3m 33s
type-checking (3.8)	36s
Image tests	40s
tests (windows-latest, 3.12)	2m 44s
type-checking (3.12)	47s
tests (ubuntu-20.04, 3.8, --constraint constraints/minimal_dependencies.txt)	59s
docs	22s
	10m 41s

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