set correct best bound, set bad nodes to :Worse

[Wikunia]

Even if not sorting it should produce the correct result

• therefore the best_bound must be set using minimum/maximum
• if a solution was found then nodes which are worse shouldn't be added
• existing nodes which are worse should be marked as such.

Currently all nodes have to be saved (they can't be removed from backtrack_vec as the id corresponds to position in the array.

[codecov[bot]]

Codecov Report

Merging #35 into master will decrease coverage by 0.85%. The diff coverage is 80.95%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage    98.9%   98.05%   -0.86%     
==========================================
  Files          10       10              
  Lines         913      924      +11     
==========================================
+ Hits          903      906       +3     
- Misses         10       18       +8

Impacted Files	Coverage Δ
src/ConstraintSolver.jl	96.52% <80.95%> (-2.46%)	:arrow_down:
src/eq_sum.jl	100% <0%> (+1.66%)	:arrow_up:

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