fix ACA bug for vector problems

[Aguelord]

Adding a sparse matrix to an HMatrix for vector problemes with the LinearAlgebra.axpy function was impossible due to a bug. It is now fixed.

Some tests were also added to check the accuracy of such a "+" operation added to the julia "Base".

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.54%. Comparing base (82eed5b) to head (990d377).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   75.18%   76.54%   +1.35%     
==========================================
  Files          14       14              
  Lines        1475     1475              
==========================================
+ Hits         1109     1129      +20     
+ Misses        366      346      -20     

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