This commit adds the virtual method for running processing algs
to the base c++ class, and adds the initial framework
for c++ algorithm parameters.
When running an algorithm, a QVariantMap is passed
as the algorithm parameters. The high level API provided
by QgsProcessingParameters should be used to retrieve
strings/layers/doubles/etc from this QVariantMap.
This allows advanced use cases, such as passing QgsProperty
with the QVariantMap for "dynamic" parameters, where the
value should be evaluated for every feature processed.
E.g. if the buffer algorithm uses a dynamic property for distance,
then the distance could be bound to either a field value or
to a custom expression. This gets evaluated before buffering
each feature to allow for advanced variable buffering.
Support for dynamic parameters will be "opt in", and non default.
So algorithms will need to specifically add support for
dynamic properties as required.
