add defaults for the several structs

src/dynamics/joint/spherical_joint.rs

@@ -7,7 +7,11 @@ use crate::math::{Point, Real};
 pub struct SphericalJoint {
     data: JointData,
 }
-
+impl Default for SphericalJoint{
+    fn default() -> Self {
+        SphericalJoint::new()
+    }
+}
 impl SphericalJoint {
     pub fn new() -> Self {
         let data =