Support for mixed precip/noprecip input and AR order 1

[ladc]

Fix several issues in case of intermittent rainfall in the radar images:

• Fix mixed rain/norain input frames
• Add tests
• Make AR1 work for regular steps
• Add tests
• Make AR1 work for steps blending
• Add tests

Works towards fixing issue #543.

[codecov]

Codecov Report

❌ Patch coverage is 93.68421% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.12%. Comparing base (29a317b) to head (55261f1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pysteps/blending/steps.py	77.77%	6 Missing ⚠️
pysteps/blending/skill_scores.py	70.00%	3 Missing ⚠️
pysteps/nowcasts/steps.py	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #544      +/-   ##
==========================================
+ Coverage   83.89%   84.12%   +0.23%     
==========================================
  Files         168      170       +2     
  Lines       14678    14980     +302     
==========================================
+ Hits        12314    12602     +288     
- Misses       2364     2378      +14     

Flag	Coverage Δ
unit_tests	84.12% <93.68%> (+0.23%)	⬆️

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[FelixE91]

We consider these fixes important enough to merge them into the master branch. They can have a direct positive impact on the operation of pysteps, as they
(i) prevent some runs from crashing when mixed time steps with and without precipitation are present in the radar inputs
(ii) provide more readable and clearer error messages if the run does abort due to mixed time steps with and without precipitation in the radar inputs
The tests (pytest) are still generating numerous warnings. Most of them are unrelated to the code we added here.
The warnings from the pytest execution should be investigated as a separate issue #545