Added derivation of hfns

[schlunma]

Description

This PR adds the derivation of hfns also used in the EMAC CMORizer in order to decrease the size of #1554.

Checklist

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[codecov]

Codecov Report

Merging #1594 (d9c14d3) into main (b902e58) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1594      +/-   ##
==========================================
+ Coverage   90.83%   90.84%   +0.01%     
==========================================
  Files         200      201       +1     
  Lines       10648    10662      +14     
==========================================
+ Hits         9672     9686      +14     
  Misses        976      976              

Impacted Files	Coverage Δ
esmvalcore/preprocessor/_derive/hfns.py	100.00% <100.00%> (ø)

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[remi-kazeroni]

Looks good to me on the tech side, thanks @schlunma!

[axel-lauer]

Looks good to me.