Lazy median aggregator

[fnattino]

🚀 Pull Request

Description

Adding the lazy implementation of the median aggregator, following the implementation discussed in #4039 .

It is true that the issue above suggests that there was not much interest for a lazy median, but it looked like it would need only little effort. And we thought it could still be useful to some ESMValTool users who might not be aware of the workaround of using the 50th percentile.

CC: @bouweandela

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.81%. Comparing base (cc60a60) to head (4ea685e).

Files with missing lines	Patch %	Lines
lib/iris/analysis/__init__.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6167      +/-   ##
==========================================
- Coverage   89.81%   89.81%   -0.01%     
==========================================
  Files          88       88              
  Lines       23174    23182       +8     
  Branches     5045     5046       +1     
==========================================
+ Hits        20814    20821       +7     
  Misses       1623     1623              
- Partials      737      738       +1     

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