Improve netCDF4 compression

[brmather]

• Add option for quantisation by specifying significant digits
• Replace default zlib compression for newer zstd compression
• Increase compression level to 9

[brmather]

Some of these compression methods are not compatible with all versions of netCDF and some of the data quantisation handled by significant_digits keyword sometimes garbles the netCDF data. More investigation is required.

[brmather]

Turns out zlib is still the best way to ensure the .nc files can be uncompressed without errors.

significant_digits does work, and will preserve nan masks so long as there is at least two significant digits.

Some timings on a 3601 x 1801 grid:

• complevel=4 = 5.6MB file in 386ms
• complevel=6 = 5.1MB file in 410 ms
• complevel=9 = 5.1MB file in 550ms
• significant_digits=2, complevel=4 = 850KB file in 300ms
• significant_digits=2, complevel=6 = 450KB file in 350ms
• significant_digits=2, complevel=9 = 311KB file in 1s

A complevel of 6 or 7 seems to be the best tradeoff between compression speed and file size. Specifying significant_digits yields a huge reduction in file size for negligible speed penalty.

[brmather]

The seafloor age gridding workflow has now been modified to use the new compression options in write_netcdf_grid. The read_netcdf_grid could have more flexible reading of .nc files. Will address that later down the track.

[jcannon-gplates]

Nice job with the compression! Not easy to compress floating-point numbers.