Shrink _recipe.py

[valeriupredoi]

I think we have reached the size and complexity limit for _recipe.py (we may have reached that a while ago) so I reckon we should start shrinking it and optimizing its use; I reckon the first step is to move all the preprocessor module specific functions (like _update_{STEP}_settings) to the preprocessor itself. What say you @bouweandela - if you agree I can get cracking with it starting now 🍺

[bouweandela]

Hi V,

I agree with you that it would be good to clean this file up. In the next few months we will be working on #498. I expect that once we have a more complete Python API in place, _recipe.py will also be a lot simpler, so I think it would be best to wait a bit with this.

Another issue is that at the moment we have about 90 open pull requests (45 in ESMValCore and 46 in ESMValTool), I would really like to focus on getting those merged before starting new work.

[valeriupredoi]

makes sense, man! let's wait a bit more, I'll dedicate my effeorts towards the noble cause of solving as many of those PR's and issues 🎠

[bouweandela]

Some of this was achieved in #1609.