Support of multiple solvers

[guimarqu]

Fix #54

[codecov]

Codecov Report

Merging #56 (ab3d108) into master (dc64d11) will increase coverage by 0.06%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   75.40%   75.46%   +0.06%     
==========================================
  Files          10       10              
  Lines         374      379       +5     
==========================================
+ Hits          282      286       +4     
- Misses         92       93       +1     

Impacted Files	Coverage Δ
src/formulations.jl	97.77% <93.33%> (-2.23%)	⬇️
src/annotations.jl	96.77% <100.00%> (ø)
src/branchingpriority.jl	100.00% <0.00%> (ø)
src/tree.jl	63.00% <0.00%> (+0.37%)	⬆️

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