Avoid mutating the input cubes when building the combined cube in preprocessor function multi_model_statistics

[bouweandela]

Description

Avoid mutating the input cubes when building the combined cube in preprocessor function multi_model_statistics.

Issue was introduced in #2518 and is not present in any released version of the tool.

Closes #2559

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.77%. Comparing base (e9fccbd) to head (b0498b3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2564   +/-   ##
=======================================
  Coverage   94.77%   94.77%           
=======================================
  Files         251      251           
  Lines       14266    14267    +1     
=======================================
+ Hits        13520    13521    +1     
  Misses        746      746           

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[bettina-gier]

Just commenting to say that this fixes the issue, the new portrait plot recipe runs successfully with this Pr as tested by @lukruh and me.