rigidBodyMeshMotion: Added optional force damping ramp function

to provide smoother behavior on start-up when an acceleration impulse is
applied, e.g. if the body is suddenly released.  e.g.

dynamicFvMesh       dynamicMotionSolverFvMesh;

motionSolverLibs   ("librigidBodyMeshMotion.so");

solver             rigidBodyMotion;

rigidBodyMotionCoeffs
{
    report          on;

    solver
    {
        type    Newmark;
    }

    ramp
    {
        type     quadratic;
        start    0;
        duration 10;
    }
.
.
.

will quadratically ramp the forces from 0 to their full values over the first
10s of the run starting from 0.  If the 'ramp' entry is omitted no force ramping
is applied.