Allow using multi model statistics preprocessor on datasets without timerange

[schlunma]

Description

Since #1808, it is possible to use the multi-model statistics preprocessor on data with arbitrary dimensions (i.e., also on data without a time dimension). Thus, it should be possible to use it on data without a timerange, too.

This PR implements that.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.06%. Comparing base (e415269) to head (7f9f33a).
Report is 46 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2644   +/-   ##
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  Coverage   95.06%   95.06%           
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  Files         255      255           
  Lines       14880    14885    +5     
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+ Hits        14146    14151    +5     
  Misses        734      734           

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[valeriupredoi]

looks good! One thing I am a bit confused by is how did the testing for the case of a dataset without time axis was done before this PR, if that dataset would not have a timerange attribute

[valeriupredoi]

nvmd, I unconfused myself - there wasn't much of that 😁