fix geopotential_height chunks

[zebengberg]

Closes #280

Fixes

Bypass the ValueError raised by dask.array.gradient when the input array is not correctly chunk along the level dimension. Previously, Cocip would raise an error when computing tau cirrus in the case that the met data had single chunks along the level dimension.

Tests

• QC test passes locally (make test)
• CI tests pass

Reviewer

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