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 94.77%. Comparing base (ffaeff1) to head (5354c15).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2518      +/-   ##
==========================================
- Coverage   94.77%   94.77%   -0.01%     
==========================================
  Files         249      249              
  Lines       14094    14092       -2     
==========================================
- Hits        13358    13356       -2     
  Misses        736      736              

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[valeriupredoi]

thanks, bud 🍺