Fix Create constant raster

[nmaarnio]

Create constant raster does not work currently. Should be matched to the toolkit CLI definition.

[lehtonenp]

@nmaarnio, any help with making the target epsg optional and use the project's epsg if it is not chosen? Should I override the processAlgorithm()?

I'm adding on to this comment: Should the CRS be optional in the Plugin?

[nmaarnio]

I think following the logic of QGIS's own Create constant raster layer is a good idea. So CRS is not optional. Overall our algorithm will be quite identical to that one, with the exception that we don't expose number type definition to users.