Lazy derivation of vegfrac

[bouweandela]

Description

Related to #2451

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[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.83%. Comparing base (4b0dd41) to head (0763861).

Files with missing lines	Patch %	Lines
esmvalcore/preprocessor/_derive/vegfrac.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2452   +/-   ##
=======================================
  Coverage   94.83%   94.83%           
=======================================
  Files         251      251           
  Lines       14191    14191           
=======================================
  Hits        13458    13458           
  Misses        733      733           

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