Avoid copying input data in CMIP6 CESM2 air pressure level fixes

[bouweandela]

Description

Use ncdata to fix files in-memory instead of copies on disk.

Related to #2129 and #674

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[valeriupredoi]

very cool @bouweandela 👏 Always nice to see data transfer optimizations, hope you don't mind me turning on the GA see if any issues for older Pythons - I took a look at the failed tests - couple of them due to mocks not called (easy fix) but one fails due to what looks to be a reversal of the lev bounds, can prob fix that for you here, if you don't have time 🍺