* add SHAP summary plot using ggplot2
* Update xgb.plot.shap
* Update example in xgb.plot.shap documentation
* update logic, add tests
* whitespace fixes
* whitespace fixes for test_helpers
* namespace for sd function
* explicitly declare variables that are automatically evaluated by data.table
* Fix R lint
Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>