Fixed clip_timerange if only a single time point is extracted

[schlunma]

Description

The default behavior of iris.cube.Cube.extract() is to return a scalar coordinate if only a single value is extracted. This causes some NCL diagnostics to fail if clip_timerange extracts only single point in time. This PR fixes this behavior.

Closes #1496

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[valeriupredoi]

cool, thanks, Manu! Just a minor note that we should check units and calendar are preserved too

[codecov]

Codecov Report

Merging #1497 (7c07230) into main (51a8a02) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1497   +/-   ##
=======================================
  Coverage   90.71%   90.72%           
=======================================
  Files         197      197           
  Lines       10517    10527   +10     
=======================================
+ Hits         9541     9551   +10     
  Misses        976      976           

Impacted Files	Coverage Δ
esmvalcore/preprocessor/_time.py	95.65% <100.00%> (+0.11%)	⬆️

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[schlunma]

nice, can you add a call to units preservation as well please?

Added!

I also added support for cases where time is not the first dimension. Never seen this in any dataset, but better safe than sorry 😄

[valeriupredoi]

sehr gut 😁 I've never seen time anywhere but the 0th dimension either