Enhancing the use of external nowcasts to also support ensembles and noise

[Joep1999]

No description provided.

[RubenImhoff]

Still in progress, but fixes #482.

[codecov]

Codecov Report

❌ Patch coverage is 34.04255% with 93 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.81%. Comparing base (b12c420) to head (3b256e9).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pysteps/blending/steps.py	32.60%	93 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
- Coverage   84.14%   83.81%   -0.34%     
==========================================
  Files         168      168              
  Lines       14507    14588      +81     
==========================================
+ Hits        12207    12227      +20     
- Misses       2300     2361      +61     

Flag	Coverage Δ
unit_tests	83.81% <34.04%> (-0.34%)	⬇️

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[RubenImhoff]

Tasks:

• Add noise generator to external nowcast setup.
• Add option to provide ensemble nowcast.
• Add option to provide deterministic nowcast, but request ensemble output.
• Add gallery example.
• Add and update tests.