era5 encoding is lost with preprocess_ERA5

[veenstrajelmer]

When reducing on non-decoded dataset (decode_cf=False upon opening), this preserves the encoding attrs:

ds_reduced = ds_raw.max(dim="expver", keep_attrs=True)

However, in preprocess_ERA5, the input dataset is already decoded and this does not help in preserving encoding (it will be dropped by the reduction). This means the reduction in this function prevents dtype int in a to_netcdf file already. Therefore, prevent_dtype_int is of no value in this function if this expver dimension is present. These files will then be written unzipped to float32 dtypes. It would be good to make this more robust and transparent.