Don't convert NWPs to `float16`!  Converting to `float16` results in infinities in `dswrf`

**Describe the bug**
Downwards short-wave radiation (dswrf) (i.e. irradiance) looks like it is mostly (entirely?) Inf when converted to float16.

**Possible fix**
For now, let's leave NWPs as float32.

If we really want to convert to float16 (or int16) then we should probably re-scale the values to better fit the target dtype.