Keep float64 when converting to xarray

[dmey]

Currently, irrespective of the original data precision, a cast to float32 is made when converting to xarray.(see https://github.com/ecmwf/cfgrib/blob/master/cfgrib/dataset.py#L332) This can however downgrade the type of grib fields stored at 64-bits. Can you please allow to specify this at conversion or to automatically match the type specified in grib fields?

[Marston]

@dmey Was this issue ever addressed? Did you find a work-around?