cat experiments first, then do the time based cat

[dhohn]

Description

Solves a bug where concatenation of multiple experiments consisting of datasets with multiple files leads wrong results, where timeranges are mixed randomly from the experiments.
Closes #2342

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Before you get started

• ☝ Create an issue concatenate experiments handles time ordering incorrectly #2342

Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 94.04%. Comparing base (9728667) to head (2eed6d1).

Files	Patch %	Lines
esmvalcore/preprocessor/_io.py	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2343      +/-   ##
==========================================
- Coverage   94.07%   94.04%   -0.03%     
==========================================
  Files         241      241              
  Lines       13414    13424      +10     
==========================================
+ Hits        12619    12625       +6     
- Misses        795      799       +4     

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[dhohn]

I need help with adding tests.