Add buffered model

[blegat]

This simplifies the use of LowRankOpt for solvers like Loraine. They don't have to deal with the buffers, they can just use the buffered model.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.36%. Comparing base (31794a1) to head (563332b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   99.41%   99.36%   -0.06%     
==========================================
  Files          21       23       +2     
  Lines        1198     1258      +60     
==========================================
+ Hits         1191     1250      +59     
- Misses          7        8       +1     

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