start with deterministic version of external nowcast blending

[Joep1999]

Started adding support for externally computed nowcasts such as ml based nowcasts like DGMR

[RubenImhoff]

Fixes #499.

[RubenImhoff]

To do:

• Review and refactor code (Ruben)
• Add tests (Avijit + Joep + Ruben)
• Gallery example? (Joep + Ruben)

[RubenImhoff]

@Joep1999, FYI for your tests, I'm changing the names from STEPS to steps and from External nowcasting to external_nowcast.

[codecov]

Codecov Report

❌ Patch coverage is 76.30058% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.95%. Comparing base (8c4e510) to head (296c50f).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
pysteps/blending/steps.py	70.92%	41 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
- Coverage   84.58%   83.95%   -0.64%     
==========================================
  Files         162      163       +1     
  Lines       13471    13739     +268     
==========================================
+ Hits        11394    11534     +140     
- Misses       2077     2205     +128     

Flag	Coverage Δ
unit_tests	83.95% <76.30%> (-0.64%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.