rigidBodyDynamics::restraints::externalForce: New restraint to apply a time-varying force

Description
    Time-dependent external force restraint using Function1.

Usage
    Example applying a constant force to the floatingObject:
    restraints
    {
        force
        {
            type        externalForce;
            body        floatingObject;
            location    (0 0 0);
            force       (100 0 0);
        }
    }

Based on code contributed by SeongMo Yeon
Resolves contribution request https://bugs.openfoam.org/view.php?id=3358

tutorials/multiphase/interFoam/RAS/floatingObject/constant/dynamicMeshDict

@@ -71,6 +71,17 @@ rigidBodyMotionCoeffs
             outerDistance   0.35;
         }
     }
+
+    restraints
+    {
+        force
+        {
+            type externalForce;
+            body floatingObject;
+            location (0 0 0);
+            force    (100 0 0);
+        }
+    }
 }