add a mini filter: h5z-clamp, with a filter ID of 45678

[shaomeng]

Filter h5z-clamp does not do anything during compression, and does only one thing during decompression: it clamps any negative value to 0.0. This filter is using an ID of 45678 right now.

Example:

• nccopy -F "VAR1,45678|32028,2690908160u" <wrf_in> <wrf_out>

Here VAR1 is filtered by h5z-clamp first, then by h5z-sperr second, during compression. In reality h5z-clamp did nothing.
During decompression, h5z-sperr reconstructs all values of the variable, and then any negative ones are filtered to be 0.0 by h5z-clamp.