Move concatenate preprocessor function to its own module

[bouweandela]

Description

Having the concatenate preprocessor function in the esmvalcore.preprocessor._io module is causing circular import issues in #2765. To keep changes to a minimum in that pull request, I created a separate pull request just to move it.

Closes #issue_number

Link to documentation:

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.31%. Comparing base (4d8fcbf) to head (5c8e475).
⚠️ Report is 66 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2766   +/-   ##
=======================================
  Coverage   95.30%   95.31%           
=======================================
  Files         259      260    +1     
  Lines       15350    15357    +7     
=======================================
+ Hits        14629    14637    +8     
+ Misses        721      720    -1     

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[schlunma]

Thanks Bouwe, looks good! Do you want to fix the 3 remaining Codacy issues? Looks pretty straightforward.

[schlunma]

(Codecov shows that 1 line is missing tests, but after clicking on it I don't see anything...so I guess this is fine)

[valeriupredoi]

(Codecov shows that 1 line is missing tests, but after clicking on it I don't see anything...so I guess this is fine)

it's complaining it can't find Head report, for some reason https://app.codecov.io/gh/ESMValGroup/ESMValCore/pull/2766?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ESMValGroup - lemme merge main and see if it fixes itself

[valeriupredoi]

cheers bud, what Manu says (also about the Codacy complaints) 🍺 I can merge at the end, when all is ready 👍

[schlunma]

Perfect, now it shows the missing line. Would it be possible to add a test for that? Should be very simple, thanks!

[bouweandela]

Thanks for taking a look @schlunma and @valeriupredoi. I fixed one more Codacy issue and added a test for the one line that was not covered. I'm not sure how to solve the two remaining Codacy issues though. You're welcome to have a go at it if you have ideas.

[valeriupredoi]

spiffy! 🍺

[schlunma]

Thanks 🚀