add defaults for the several structs

src/dynamics/joint/fixed_joint.rs

@@ -6,7 +6,11 @@ use crate::math::{Isometry, Point, Real};
 pub struct FixedJoint {
     data: JointData,
 }
-
+impl Default for FixedJoint{
+    fn default() -> Self {
+        FixedJoint::new()
+    }
+}
 impl FixedJoint {
     pub fn new() -> Self {
         #[cfg(feature = "dim2")]