Discretization variable selection scheme

[jac0320]

Two algorithmic scheme is added to choose discretization variables,

• Consider minimum vertex cover method when there are more than 15 variables in the nonlinear terms, select all o/w
• Use a weighted minimum cover method to update the selection at every iteration of the AMP algorithm

This branch also contains updated tests and several small fixes.

[codecov[bot]]

Codecov Report

Merging #71 into master will increase coverage by 2.16%. The diff coverage is 55.6%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   65.55%   67.71%   +2.16%     
==========================================
  Files          26       27       +1     
  Lines        4503     5957    +1454     
==========================================
+ Hits         2952     4034    +1082     
- Misses       1551     1923     +372

Impacted Files	Coverage Δ
test/algorithm.jl	100% <ø> (ø)	:arrow_up:
examples/nlp.jl	89.28% <ø> (ø)	:arrow_up:
examples/circle.jl	35% <ø> (ø)	:arrow_up:
src/amp.jl	65.44% <ø> (+4.41%)	:arrow_up:
examples/multi.jl	34.4% <ø> (ø)	:arrow_up:
src/bounds.jl	71.51% <ø> (+29.74%)	:arrow_up:
examples/blend146.jl	0% <0%> (ø)
src/presolve.jl	88.63% <100%> (ø)	:arrow_up:
src/algorithm.jl	84.9% <100%> (+0.05%)	:arrow_up:
src/log.jl	74.76% <100%> (+41.43%)	:arrow_up:
... and 13 more

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