Add check for delta unit to convert #1082

Job	Run time
test-windows (3.9, numpy>=1.21,<2.0.0)	55s
test-macos (3.9)	2m 30s
test-windows (3.10, numpy>=1.21,<2.0.0)	1m 6s
test-linux (3.9)	1m 34s
test-macos (3.9, numpy>=1.21,<2.0.0)	3m 8s
test-windows (3.11, numpy>=1.21,<2.0.0)	57s
test-macos (3.10)	2m 0s
test-linux (3.9, uncertainties==3.1.6)	1m 41s
test-macos (3.10, numpy>=1.21,<2.0.0)	2m 9s
test-linux (3.9, uncertainties>=3.1.6,<4.0.0)	2m 4s
test-linux (3.9, numpy>=1.21,<2.0.0)	1m 53s
test-macos (3.11)	1m 57s
test-macos (3.11, numpy>=1.21,<2.0.0)	2m 19s
test-linux (3.9, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	2m 4s
test-linux (3.9, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	2m 8s
test-linux (3.10)	1m 35s
test-linux (3.10, uncertainties==3.1.6)	1m 40s
test-linux (3.10, uncertainties>=3.1.6,<4.0.0)	1m 47s
test-linux (3.10, numpy>=1.21,<2.0.0)	1m 50s
test-linux (3.10, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	2m 6s
test-linux (3.10, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	2m 10s
test-linux (3.11)	1m 26s
test-linux (3.11, uncertainties==3.1.6)	1m 40s
test-linux (3.11, uncertainties>=3.1.6,<4.0.0)	1m 38s
test-linux (3.11, numpy>=1.21,<2.0.0)	1m 48s
test-linux (3.11, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	2m 16s
test-linux (3.11, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	1m 55s
test-linux (3.9, numpy, matplotlib==2.2.5)	2m 35s
test-linux (3.9, numpy, uncertainties, sparse xarray netCDF4 dask[complete]==2023.4.0 graphviz ba...	2m 45s
	55m 36s

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