Instead of returning a single QgsFeature from processFeature, we now
return a list of features.
This allows feature based algorithms which return multiple features
per input feature, e.g. "explode" type algorithms which split a
single input feature into multiple output features.
Allows upgrading geometries to include these dimensions, or
overwriting any existing Z/M values with a new value.
Intended mostly as a test run for QgsProcessingFeatureBasedAlgorithm
