Add cloud scatterplots

[bouweandela]

Description

Closes #195

Sample data available in Climate-REF/ref-sample-data#30

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
...rc/cmip_ref_metrics_esmvaltool/metrics/__init__.py	100.00% <100.00%> (ø)
...ol/src/cmip_ref_metrics_esmvaltool/metrics/base.py	100.00% <100.00%> (ø)
...altool/metrics/climate_at_global_warming_levels.py	100.00% <100.00%> (ø)
...f_metrics_esmvaltool/metrics/cloud_scatterplots.py	100.00% <100.00%> (ø)
...ool/src/cmip_ref_metrics_esmvaltool/metrics/ecs.py	100.00% <100.00%> (ø)
...src/cmip_ref_metrics_esmvaltool/metrics/example.py	100.00% <100.00%> (ø)
..._esmvaltool/metrics/sea_ice_area_seasonal_cycle.py	100.00% <100.00%> (ø)
...ool/src/cmip_ref_metrics_esmvaltool/metrics/tcr.py	100.00% <100.00%> (ø)
...ol/src/cmip_ref_metrics_esmvaltool/metrics/tcre.py	100.00% <100.00%> (ø)
...ool/src/cmip_ref_metrics_esmvaltool/metrics/zec.py	100.00% <100.00%> (ø)
... and 1 more

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[bouweandela]

I had to switch to pyyaml because the recipe uses multiple << keys to insert values into a mapping and ruamel.yaml either throws and error or if you tell it to ignore the error, silently discards the previous key.

[bouweandela]

This dataset cannot be ingested with the REF #260 so I included it here.