At the moment, the preprocessor multi_model_statistics will also attempt to automatically 'regrid' the time coordinate of the incoming datasets. Other coordinates, such as horizontal and vertical coordinates, need to be explicitly regridded by defining a preprocesor in the recipe. It would be good to be consistent and make the temporal regridding more visible and move the temporal regridding to the regrid_time preprocessor. If the datasets have a time coordinate, users would then need to specify the regrid_time preprocessor in the recipe as well as as the regrid and extract_levels.

This would also fix the problem described in #738, that multi_model_statistics does not work for variables that do not have a time coordinate.