add node directly with default empty BnBNodeInfo

[Wikunia]

Implements #37

I'm not sure whether this approach is a reasonable one. There is a default BnBNodeInfo now which is used by a default constructor of an AbstractNode. The values of id, lb and ub are then updated when the node is actually added to the tree.

All docstrings need fixes but maybe you (@matbesancon ) can already say whether this solves the flexibility problem you had in mind.

[codecov[bot]]

Codecov Report

Merging #38 (a849f84) into main (10f0b93) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #38   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          131       133    +2     
=========================================
+ Hits           131       133    +2     

Impacted Files	Coverage Δ
src/Bonobo.jl	100.00% <100.00%> (ø)
src/node.jl	100.00% <100.00%> (ø)

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