QgsExpressionContextGenerator and QgsProcessingContextGenerator

[Benjamin Jakimow]

QgsExpressionContext and QgsProcessingContext are widely used by many widgets and routines in the QGIS API.

The EnMAP-Box could use these, e.g. to provide a context + scope variables that consider the current state of the EnMAP-Box GUI, for example the current map canvas, current layer and other variables.

I recommend that the EnMAP-Box implements the QgsExpressionContextGenerator and QgsProcessingContextGenerator interfaces to provide these context objects.

[Benjamin Jakimow]

Impemented

EnMAPBox().createExpressionContext()
EnMAPBox().processingContext()

unit test added to tests/src/test_enmapbox.py::test_context_interfaces(self)