Avoid AbstractVector in stats.solver_specific

[tmigot]

Connected to #101 , we do not keep AbstractVector from stats.solver_specific in the output of solver_problems (and therefore bmark_solvers)

[codecov]

Codecov Report

Merging #102 (f5f82ea) into main (be8d2f1) will increase coverage by 0.64%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
+ Coverage   77.17%   77.81%   +0.64%     
==========================================
  Files          13       13              
  Lines         311      311              
==========================================
+ Hits          240      242       +2     
+ Misses         71       69       -2     

Impacted Files	Coverage Δ
src/run_solver.jl	86.84% <33.33%> (+2.63%)	⬆️
src/formats.jl	76.92% <0.00%> (+3.84%)	⬆️

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[abelsiqueira]

Looks good to me, thanks for the PR.