Fixes to linear and saliency based blending (pySTEPS#466)

* Fix: ensure linear blending can also handle multiple input timesteps and ensure linear blending can deal with nans.

* add new release

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Co-authored-by: mats-knmi <[email protected]>

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Fix deprecated arg name in example

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Update RainFARM example following pySTEPS#311 (pySTEPS#367)

* Update RainFARM example following pySTEPS#311

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