add new keyword argument rw_weight to optimize_tree

[GiggleLiu]

No description provided.

[codecov]

Codecov Report

Merging #12 (bec18fb) into master (9198b96) will increase coverage by 0.61%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   94.20%   94.82%   +0.61%     
==========================================
  Files           3        3              
  Lines         449      464      +15     
==========================================
+ Hits          423      440      +17     
+ Misses         26       24       -2     

Impacted Files	Coverage Δ
src/treesa.jl	94.17% <96.29%> (+1.50%)	⬆️

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