del model in map_function

[braingram]

This PR makes a minor change to AbstractModelLibrary.map_function to reduce memory usage. More details can be found in the issue but the tldr is that this prevents having 2x models in memory during map_function.

Fixes: #173

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.29%. Comparing base (b66eb3f) to head (5efae61).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
+ Coverage   73.27%   73.29%   +0.01%     
==========================================
  Files          25       25              
  Lines        1916     1917       +1     
==========================================
+ Hits         1404     1405       +1     
  Misses        512      512              

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[braingram]

Thanks! The romancal failure is due to the unrelated background schema changes.