added Triangle and Tetgen as submodules

Former-commit-id: 59475b3fa380a2551d34d3d1991ada4b8b7570fc

.gitmodules

@@ -9,3 +9,9 @@ url=https://github.com/libigl/nanogui.git
 [submodule "external/pybind11"]
 	path = external/pybind11
 	url = https://github.com/wjakob/pybind11.git
+[submodule "external/tetgen"]
+	path = external/tetgen
+	url = https://github.com/libigl/tetgen.git
+[submodule "external/triangle"]
+	path = external/triangle
+	url = https://github.com/libigl/triangle.git