Implement unit conversion for contour levels #397

Job	Run time
initial_checks / Load tox environments	5s
initial_checks / codestyle (ubuntu-latest)	20s
no_coverage / Load tox environments	4s
tests / Load tox environments	9s
no_coverage / py39-notebooks (windows-latest)	3m 8s
no_coverage / py38-notebooks (macos-latest)	3m 34s
no_coverage / py310-notebooks (ubuntu-latest)	1m 48s
no_coverage / py311-docs (ubuntu-latest)	1m 27s
no_coverage / py39-docs (macos-latest)	3m 6s
no_coverage / py38-docs (windows-latest)	2m 51s
tests / py38-test (ubuntu-latest)	2m 13s
tests / py39-test (ubuntu-latest)	2m 24s
tests / py310-test (ubuntu-latest)	2m 18s
tests / py311-test (ubuntu-latest)	2m 16s
tests / py38-test (macos-latest)	3m 53s
tests / py39-test (macos-latest)	5m 54s
tests / py311-test (macos-latest)	7m 28s
tests / py38-test (windows-latest)	3m 6s
tests / py39-test (windows-latest)	3m 24s
tests / py310-test (windows-latest)	3m 13s
publish / Build source and wheel distribution	4m 7s
	56m 48s

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