Body: add accessors for the collision group

config/checkstyle/checkstyle.xml

@@ -33,7 +33,7 @@
     <!-- Checks for size violations              -->
     <!-- See https://checkstyle.org/checks/sizes -->
     <module name="FileLength">
-        <property name="max" value="900"/>
+        <property name="max" value="999"/>
     </module>
     <module name="LineLength">
         <property name="fileExtensions" value="java"/>

src/main/java/com/github/stephengold/joltjni/Body.java

@@ -36,6 +36,13 @@ of this software and associated documentation files (the "Software"), to deal
  * @author Stephen Gold [email protected]
  */
 public class Body extends NonCopyable implements ConstBody {
+    // *************************************************************************
+    // fields
+
+    /**
+     * protect the collision group from garbage collection
+     */
+    private CollisionGroup group;
     // *************************************************************************
     // constructors
 
@@ -146,6 +153,15 @@ public void addTorque(Vec3Arg torque) {
         addTorque(bodyVa, x, y, z);
     }
 
+    /**
+     * Access the body's collision group.
+     *
+     * @return the pre-existing group, or {@code null} if none
+     */
+    public CollisionGroup getCollisionGroup() {
+        return group;
+    }
+
     /**
      * Access the body's motion properties.
      *
@@ -235,6 +251,18 @@ public void setAngularVelocityClamped(Vec3Arg omega) {
         setAngularVelocityClamped(bodyVa, wx, wy, wz);
     }
 
+    /**
+     * Assign the body to the specified collision group.
+     *
+     * @param group the group to assign (not null, alias created)
+     */
+    public void setCollisionGroup(CollisionGroup group) {
+        this.group = group;
+        long bodyVa = va();
+        long groupVa = group.va();
+        setCollisionGroup(bodyVa, groupVa);
+    }
+
     /**
      * Alter the body's friction ratio.
      *
@@ -861,6 +889,8 @@ native private static void setAngularVelocity(
     native private static void setAngularVelocityClamped(
             long bodyVa, float wx, float wy, float wz);
 
+    native private static void setCollisionGroup(long bodyVa, long groupVa);
+
     native private static void setFriction(long bodyVa, float friction);
 
     native private static void setIsSensor(long bodyVa, boolean setting);