Allow validate to work with scenario outputs

[Martin-Jung]

Currently the validate() function only works with fitted models and independently supplied raster layers.
Ideally it is also possible to validate a scenario objects.
In this case validation points per time step (set as another column) have to be provided as input.
This functionality could be quite helpful in cases scenarios are back-casted (e.g. use years 2000 to 2005 data to project and validate for years 2005-2020).

Outputs should be overall averages and well average per time step.