Don't include the gz/math.hh header from library code (#1043)

Using overly-broad include statements leads to slower builds and
bloated object code.

Signed-off-by: Jeremy Nimmer <[email protected]>

Author: jwnimmer-tri

include/sdf/Link.hh

@@ -19,6 +19,7 @@
 
 #include <memory>
 #include <string>
+#include <ignition/math/Inertial.hh>
 #include <ignition/math/Pose3.hh>
 #include "sdf/Element.hh"
 #include "sdf/SemanticPose.hh"

include/sdf/Param.hh

@@ -30,7 +30,12 @@
 #include <variant>
 #include <vector>
 
-#include <ignition/math.hh>
+#include <ignition/math/Angle.hh>
+#include <ignition/math/Color.hh>
+#include <ignition/math/Pose3.hh>
+#include <ignition/math/Quaternion.hh>
+#include <ignition/math/Vector2.hh>
+#include <ignition/math/Vector3.hh>
 
 #include "sdf/Console.hh"
 #include "sdf/sdf_config.h"

src/parser_urdf.cc

@@ -25,7 +25,10 @@
 #include <utility>
 #include <vector>
 
-#include <ignition/math.hh>
+#include <ignition/math/Helpers.hh>
+#include <ignition/math/Pose3.hh>
+#include <ignition/math/Quaternion.hh>
+#include <ignition/math/Vector3.hh>
 
 #include <urdf_model/model.h>
 #include <urdf_model/link.h>