Indicate cubic memory overhead in generate_precomputes docstring

[matt-graham]

Resolves #246

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.07%. Comparing base (8f6e4d5) to head (bfc9a39).
Report is 39 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #257   +/-   ##
=======================================
  Coverage   96.07%   96.07%           
=======================================
  Files          31       31           
  Lines        3565     3565           
=======================================
  Hits         3425     3425           
  Misses        140      140           

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[CosmoMatt]

Seems reasonable (and correct of course). We really were a little too lax on the numpy development, again the reason (excuse) being that our focus was on the JAX side of things!