Performance improvement for xgboost exporter

[Nirmal-Neel]

The way of getting the tree dump from the model is changed.

[codecov-commenter]

Codecov Report

Merging #50 (e463bf1) into master (1e5761f) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   97.23%   97.21%   -0.03%     
==========================================
  Files           9        9              
  Lines        1699     1685      -14     
==========================================
- Hits         1652     1638      -14     
  Misses         47       47              

Impacted Files	Coverage Δ
nyoka/xgboost/xgboost_to_pmml.py	99.30% <100.00%> (-0.04%)	⬇️
nyoka/skl/skl_to_pmml.py	98.14% <0.00%> (-0.02%)	⬇️
nyoka/statsmodels/statsmodels_to_pmml.py	98.93% <0.00%> (-0.01%)	⬇️
nyoka/statsmodels/exponential_smoothing.py	84.53% <0.00%> (ø)

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