sixDoFSolver: Run-time selectable solver (integrator) for sixDoFRigidBodyMotion

The built-in explicit symplectic integrator has been replaced by a
general framework supporting run-time selectable integrators.  Currently
the explicit symplectic, implicit Crank-Nicolson and implicit Newmark
methods are provided, all of which are 2nd-order in time:

Symplectic 2nd-order explicit time-integrator for 6DoF solid-body motion:

    Reference:
        Dullweber, A., Leimkuhler, B., & McLachlan, R. (1997).
        Symplectic splitting methods for rigid body molecular dynamics.
        The Journal of chemical physics, 107(15), 5840-5851.

    Can only be used for explicit integration of the motion of the body,
    i.e. may only be called once per time-step, no outer-correctors may be
    applied.  For implicit integration with outer-correctors choose either
    CrankNicolson or Newmark schemes.

    Example specification in dynamicMeshDict:
    solver
    {
        type    symplectic;
    }

Newmark 2nd-order time-i...