ToQUBO Compiler Upgrade

[pedromxavier]

This PR aims to make the reformulation modular, helping the development and testing of its current and subsequent features.

The necessary tasks to merge it are:

• Fix the quadratization issues (closes NTR_KZFD quadratization dispatch not being called #69)
• Establish the Compiler module (closes Place toqubo_* methods inside new Compiler module #75)
• Consolidate the Attribute System (closes Enhance and Validate the Attribute System #78, closes Add hint attributes for penalty factors #79)

[codecov]

Codecov Report

Attention: 144 lines in your changes are missing coverage. Please review.

Comparison is base (ac8be79) 70.56% compared to head (96b02fd) 80.92%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #77       +/-   ##
===========================================
+ Coverage   70.56%   80.92%   +10.35%     
===========================================
  Files          24       31        +7     
  Lines        1247     1143      -104     
===========================================
+ Hits          880      925       +45     
+ Misses        367      218      -149     

Files	Coverage Δ
src/ToQUBO.jl	100.00% <ø> (ø)
src/compiler/compiler.jl	100.00% <100.00%> (+17.02%)	⬆️
src/compiler/objective.jl	100.00% <100.00%> (ø)
src/encoding/encoding.jl	100.00% <100.00%> (ø)
src/encoding/variables/interval/bounded.jl	100.00% <100.00%> (ø)
src/model/qubo.jl	76.27% <100.00%> (+1.69%)	⬆️
src/virtual/model.jl	100.00% <100.00%> (ø)
src/virtual/variable.jl	100.00% <100.00%> (ø)
src/virtual/virtual.jl	100.00% <100.00%> (ø)
src/compiler/build.jl	90.62% <96.42%> (+9.26%)	⬆️
... and 20 more

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[bernalde]

Massive PR with many improvements. Good work. Many of the tests, if I understand correctly, were just moved to other packages.
My comments are really minor. Well done Pedro!