[TREE] add interaction constraints (#3466)

* add interaction constraints

* enable both interaction and monotonic constraints at the same time

* fix lint

* add R test, fix lint, update demo

* Use dmlc::JSONReader to express interaction constraints as nested lists; Use sparse arrays for bookkeeping

* Add Python test for interaction constraints

* make R interaction constraints parameter based on feature index instead of column names, fix R coding style

* Fix lint

* Add BlueTea88 to CONTRIBUTORS.md

* Short circuit when no constraint is specified; address review comments

* Add tutorial for feature interaction constraints

* allow interaction constraints to be passed as string, remove redundant column_names argument

* Fix typo

* Address review comments

* Add comments to Python test

doc/conf.py

@@ -41,7 +41,7 @@ sys.path.insert(0, curr_path)
 
 # -- mock out modules
 import mock
-MOCK_MODULES = ['numpy', 'scipy', 'scipy.sparse', 'sklearn', 'matplotlib', 'pandas', 'graphviz']
+MOCK_MODULES = ['scipy', 'scipy.sparse', 'sklearn', 'pandas']
 for mod_name in MOCK_MODULES:
   sys.modules[mod_name] = mock.Mock()
 
@@ -62,6 +62,8 @@ release = xgboost.__version__
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom ones
 extensions = [
+    'matplotlib.sphinxext.only_directives',
+    'matplotlib.sphinxext.plot_directive',
     'sphinx.ext.autodoc',
     'sphinx.ext.napoleon',
     'sphinx.ext.mathjax',
@@ -69,6 +71,11 @@ extensions = [
     'breathe'
 ]
 
+graphviz_output_format = 'png'
+plot_formats = [('svg', 300), ('png', 100), ('hires.png', 300)]
+plot_html_show_source_link = False
+plot_html_show_formats = False
+
 # Breathe extension variables
 breathe_projects = {"xgboost": "doxyxml/"}
 breathe_default_project = "xgboost"