consistently export data as float64

[d-v-b]

avoid cf convention / zarr v3 scale-offset filter issues by saving converted data as float64

[emmanuelmathot]

Actually, the original data are in float32. We could keep that type.

[j08lue]

Discussed in

• Request for Standardized FixedScaleOffset Codec Extension zarr-developers/zarr-extensions#42

[emmanuelmathot]

zarr-developers/zarr-extensions#43

[d-v-b]

this can be closed -- as of #134, we are consistently exporting reflectances as float32, which makes sense given the nature of that measurement.