Lazy derive preprocessor function

[bouweandela]

The derive preprocessor function does not (fully) support lazy derivation of the following variables

• toz - used in recipe_perfmetrics_CMIP5.yml, recipe_eyring06jgr.yml
• xco2 - used in recipe_gier2020bg.yml
• xch4 - used in recipe_mpqb_xch4.yml
• co2s - used in schlund20jgr/recipe_*.yml, recipe_gier2020bg.yml, recipe_wenzel16nat.yml
• uajet - used in recipe_wenzel16jclim.yml
• vegfrac - not used in any recipe Lazy derivation of vegfrac #2452

All of these derivation functions can also be 'tested' using examples/recipe_preprocessor_derive_test.yml.

Previous discussion on this topic in #42.

[bouweandela]

@chrisbillowsMO This would be a good issue to pick up if you're interested in doing Dask related work. While these derivation functions may seem a bit niche, they are used by some of the more computationally heavy recipes and they prevent is from running those with the distributed scheduler.

CC @ehogan

[chrisbillowsMO]

@chrisbillowsMO This would be a good issue to pick up if you're interested in doing Dask related work. While these derivation functions may seem a bit niche, they are used by some of the more computationally heavy recipes and they prevent is from running those with the distributed scheduler.

CC @ehogan

Thanks for thinking of me @bouweandela! I will take a look as soon as I get a chance.

[bouweandela]

Since some of these preprocessor functions predate the requirement of having unit tests, we usually test them manually by running examples/recipe_preprocessor_derive_test.yml and checking if the output is identical before and after we make changes.