Add Dask graph debug info

[bouweandela]

Description

Some preprocessor functions create Dask graphs that are too large and overwhelm the scheduler.

An example of a pattern that creates this problem is using Python to iterate over many small parts of the input array and combining the result using dask.array.stack.

This pull request adds a debug/warning message containing the graph size and asks users to report the issue if they run into it.

This should help us discover issues like #2179 and SciTools/iris#5455.

Related to #2184

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[codecov]

Codecov Report

Merging #2199 (9e77718) into main (09e20d4) will decrease coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main    #2199      +/-   ##
==========================================
- Coverage   93.14%   93.13%   -0.01%     
==========================================
  Files         237      237              
  Lines       12829    12836       +7     
==========================================
+ Hits        11949    11955       +6     
- Misses        880      881       +1     

Files Changed	Coverage Δ
esmvalcore/preprocessor/_io.py	87.07% <88.88%> (-0.04%)	⬇️