Merge input cubes only once when computing lazy multimodel statistics

[bouweandela]

Description

Merging the input cubes to the multimodel_statistics preprocessor function separately for every statistic computed is time-consuming and unnecessary. This pull request changes the code so the merge is performed only once if at least one input cube has lazy data.

This will help with making recipes run faster in #2316.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.83%. Comparing base (4b0dd41) to head (a6227c2).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2518      +/-   ##
==========================================
- Coverage   94.83%   94.83%   -0.01%     
==========================================
  Files         251      251              
  Lines       14191    14189       -2     
==========================================
- Hits        13458    13456       -2     
  Misses        733      733              

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[valeriupredoi]

thanks, bud 🍺