Add linear API

[tmigot]

No description provided.

[codecov]

Codecov Report

Merging #284 (e89575d) into main (c9e937c) will increase coverage by 2.57%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
+ Coverage   84.61%   87.18%   +2.57%     
==========================================
  Files           5        5              
  Lines         598      718     +120     
==========================================
+ Hits          506      626     +120     
  Misses         92       92              

Impacted Files	Coverage Δ
src/CUTEst.jl	84.44% <100.00%> (+1.11%)	⬆️
src/julia_interface.jl	99.71% <100.00%> (+0.12%)	⬆️

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[tmigot]

This is ready for another review. Thanks for the comments, I think I fixed most of them and tried to improve.

[dpo]

Looking good, thank you! The worry is that all those loops are slow, but currently, there's no other way.