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.
But always prefer case-exact matches for parameter names. Turns
out the grass provider requires use of parameters with the same
name but different case, so we need to be able to handle this.
Since the underlying issues with the Python bindings are now fixed,
in most cases we can safely default to allowing an algorithm to
run in a background thread!!
So now we make this the default, and require individual algorithms
which are NOT thread safe to declare this. This includes algorithms
which directly manipulate the current project or layers (such as
setting layer styles), alter the selections in layers, or which
rely on 3rd party libraries (for now, SAGA and GRASS algorithms
are marked as not thread safe... TODO - someone more familiar with
these libraries can investigate and remove the flag if appropriate).
Also models are marked as non-thread safe. TODO: only flag an
individual model as thread-unsafe if any of its child algorithms
report this flag.
