[R] Monotonic Constraints in Tree Construction (#1557)

* fix cran check

* change required R version because of utils::globalVariables

* temporary commit, monotone not working

* fix test

* fix doc

* fix doc

R-package/tests/testthat/test_monotone.R

@@ -0,0 +1,24 @@
+require(xgboost)
+
+context("monotone constraints")
+
+set.seed(1024)
+x = rnorm(1000, 10)
+y = -1*x + rnorm(1000, 0.001) + 3*sin(x)
+train = matrix(x, ncol = 1)
+
+
+test_that("monotone constraints for regression", {
+  bst = xgboost(data = train, label = y, max_depth = 2,
+                 eta = 0.1, nthread = 2, nrounds = 100,
+                 monotone_constraints = -1)
+  
+  pred = predict(bst, train)
+  
+  ind = order(train[,1])
+  pred.ord = pred[ind]
+  expect_true({
+    !any(diff(pred.ord) > 0)
+  }, "Monotone Contraint Satisfied")
+  
+})