modstat_nc: add optional zstandard support

[fjansson]

new namelist variable NAMNETCDFSTATS/zstandard, default = 0 meaning off. If zstandard is non-zero, enable zstandard compression for netcdf. If zstandard is set, it takes precedence over deflate, which is enabled by default.

Requirements:

• netcdf4 >~ 4.9.2 with zstandard support
• netcdf-fortran >= 4.6.2
• netcdf-fortran installed with the "plugin directory" set.
• DALES compiled with the cmake flag -DENABLE_ZSTANDARD=ON
• saving to the netcdf4 format (lclassic = .false.),

With the cmake flag set, netcdf-fortran >= 4.6.2
is required at compile time. If zstandard is set in the namelist, the other conditions are required,
otherwise a run-time error will occur.

[CasparJungbacker]

What happens if the plugin directory for netcdf-fortran isn't set? Does that lead to a compilation error?

[fjansson]

My understanding is this:

• without -DENABLE_ZSTANDARD=ON on the cmake command line, everything works as before, no special requirements on netcdf-fortran

• with -DENABLE_ZSTANDARD=ON, netcdf-fortran >= 4.6.2 is needed to compile (because then it contains the zstandard function). It doesn't matter how it's been compiled or if zstandard will actually work.

• when DALES has been compiled with -DENABLE_ZSTANDARD=ON, and is run with the zstandard option set to non-zero in the namelist, the zstandard function is called. This requires that netcdf-fortran has been compiled correctly with the plugin directory set to the same path as for the netcdf-c library, and that netcdf-c has been compiled with zstandard support. Otherwise it gives a runtime error.