Lazy derivation of ``vegfrac``

[bouweandela]

Description

Related to #2451

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Checklist

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.62%. Comparing base (ee16a95) to head (d92982a).

Files	Patch %	Lines
esmvalcore/preprocessor/_derive/vegfrac.py	20.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2452 +/- ## ======================================= Coverage 94.62% 94.62% ======================================= Files 246 246 Lines 14055 14055 ======================================= Hits 13299 13299 Misses 756 756 ```

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