add defaults for the several structs

src/dynamics/solver/delta_vel.rs

@@ -3,7 +3,7 @@ use na::{DVectorSlice, DVectorSliceMut};
 use na::{Scalar, SimdRealField};
 use std::ops::{AddAssign, Sub};
 
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, Default)]
 #[repr(C)]
 //#[repr(align(64))]
 pub struct DeltaVel<N: Scalar + Copy> {

src/dynamics/solver/joint_constraint/joint_generic_velocity_constraint.rs

@@ -29,6 +29,12 @@ pub struct JointGenericVelocityConstraint {
     pub writeback_id: WritebackId,
 }
 
+impl Default for JointGenericVelocityConstraint {
+    fn default() -> Self {
+        JointGenericVelocityConstraint::invalid()
+    }
+}
+
 impl JointGenericVelocityConstraint {
     pub fn invalid() -> Self {
         Self {
@@ -313,6 +319,11 @@ pub struct JointGenericVelocityGroundConstraint {
 
     pub writeback_id: WritebackId,
 }
+impl Default for JointGenericVelocityGroundConstraint{
+    fn default() -> Self {
+        JointGenericVelocityGroundConstraint::invalid() 
+    }
+}
 
 impl JointGenericVelocityGroundConstraint {
     pub fn invalid() -> Self {