initAlgorithm() method
This allows 2 benefits:
- algorithms can be subclassed and have subclasses add additional
parameters/outputs to the algorithm. With the previous approach
of declaring parameters/outputs in the constructor, it's not
possible to call virtual methods to add additional parameters/
outputs (since you can't call virtual methods from a constructor).
- initAlgorithm takes a variant map argument, allowing the algorithm
to dynamically adjust its declared parameters and outputs according
to this configuration map. This potentially allows model algorithms which
can be configured to have variable numbers of parameters and
outputs at run time. E.g. a "router" algorithm which directs
features to one of any number of output sinks depending on some
user configured criteria.
Intended as a registry for the various processing components,
including providers, algorithms and various parameters and outputs.
Currently handles only registration of providers, as a step toward
removing processing's algList (this requires first porting
the algorithm class to c++)
A QgsProcessingRegistry instance is attached to QgsApplication,
and is retrieved by QgsApplication::processingRegistry()
