Adding minNode functionality of 'ranger' and 'Rborist'

[hadjipantelis]

Both 'ranger' and 'Rborist' offer the option to control the minimum node size. This is convenient to control under/over-fitting in a natural way. All test in RegressionTests/Code work fine.

Side note: One of the test_reg_rand folds in Rborist.R produced a warning because when splitting a very small sample for CV one of the folds is too small and Rborist erred - this run produced an NA but I think it is a "good thing"; it is irrational to use very strong regularisation with very small training sets (<= 20).

[codecov-io]

Codecov Report

Merging #732 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
- Coverage   16.97%   16.97%   -0.01%     
==========================================
  Files          90       90              
  Lines       13185    13187       +2     
==========================================
  Hits         2238     2238              
- Misses      10947    10949       +2

Impacted Files	Coverage Δ
R/knn3.R	57.74% <0%> (-0.83%)	⬇️

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[topepo]

Looks good. Can you add an exit to the NEWS.md to document the changes?

[hadjipantelis]

Cool. Yes; NEWS.md updated.

Side-note: NEWS.md does not contain any updates about the (re-)inclusion of the mxnet* models. Should it be mentioned too?

[topepo]

Side-note: NEWS.md does not contain any updates about the (re-)inclusion of the mxnet* models. Should it be mentioned too?

Yes, please. I got those in at the end and didn't document it.

[hadjipantelis]

No problem; done. Also added some missing ..

[topepo]

Thanks!