The climate_statistics preprocessor sometimes changes the dtype

[zklaus]

Describe the bug
It has been reported that the multi-model statistics can have a problem with inconsistent dtypes (see ESMValGroup/ESMValTool#2218 (comment) and ESMValGroup/ESMValTool#2198 (comment)).

This seems, however, to originate a bit higher up in the processing chain. In this case, the climate_statistics preprocessor sometimes changes the dtype of the incoming cube, leading to inconsistent dtypes in the input to the multi-model statistics.

Probably the best way to address this is to fix the climate_statistics preprocessor to produce output with a consistent dtype.