Investigate further chunking improvements for better GWAS performance

[tomwhite]

#454 helped with GWAS performance, but as mentioned in https://github.com/pystatgen/sgkit/issues/390#issuecomment-768411149, there is scope for further improvement since the transfer time is still a significant proportion of the compute time.

[tomwhite]

See https://github.com/pystatgen/sgkit/issues/448#issuecomment-780655217 for the best chunk size found so far