Add SHAP interaction effects, fix minor bug, and add cox loss (#3043)

* Add interaction effects and cox loss

* Minimize whitespace changes

* Cox loss now no longer needs a pre-sorted dataset.

* Address code review comments

* Remove mem check, rename to pred_interactions, include bias

* Make lint happy

* More lint fixes

* Fix cox loss indexing

* Fix main effects and tests

* Fix lint

* Use half interaction values on the off-diagonals

* Fix lint again

tests/travis/run_test.sh

@@ -103,7 +103,7 @@ if [ ${TASK} == "cmake_test" ]; then
 
     # Build/test without AVX
     mkdir build && cd build
-    cmake .. -DGOOGLE_TEST=ON 
+    cmake .. -DGOOGLE_TEST=ON
     make
     cd ..
     ./testxgboost