Fixes to linear and saliency based blending

[RubenImhoff]

Fixes #467

PR to ensure that linear blending method can also handle multiple timesteps in the precip input variable, and to ensure that saliency-based blending works well when there are nans in the forecast.

• Ensure linear blending can also handle multiple input timesteps.
• Ensure that saliency-based blending works well when there are nans in the forecast. See the figure below from our Gallery, where it becomes clear that the saliency-based blending at +80 min becomes directly the same as NWP, while that is not warranted, as this is the time where the saliency-based blending should have just started. This seems to be caused by nans following from the _get_ranked_salience function.
• Add tests (if needed).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.56%. Comparing base (bc54616) to head (3dbde10).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #466   +/-   ##
=======================================
  Coverage   84.55%   84.56%           
=======================================
  Files         162      162           
  Lines       13456    13464    +8     
=======================================
+ Hits        11378    11386    +8     
  Misses       2078     2078           

Flag	Coverage Δ
unit_tests	84.56% <100.00%> (+<0.01%)	⬆️

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.