Add genotypes_matrix performance benchmark

[szhan]

Part of #678

[codecov]

Codecov Report

Merging #2464 (047e4d8) into main (da97323) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2464   +/-   ##
=======================================
  Coverage   93.43%   93.43%           
=======================================
  Files          28       28           
  Lines       27400    27400           
  Branches     1255     1255           
=======================================
  Hits        25600    25600           
  Misses       1766     1766           
  Partials       34       34           

Flag	Coverage Δ
c-tests	92.24% <ø> (ø)
lwt-tests	89.05% <ø> (ø)
python-c-tests	71.17% <ø> (ø)
python-tests	98.95% <ø> (ø)

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[benjeffery]

Thanks @szhan. I've modified the benchmark to use a sensible amount of RAM and added data for all previous releases.
Interestingly this confirms that the recent variant.decode work was a good decision!