unction G(x,y,t),
      and the initial condition H(x,y,t),
      the user has to supply the functions,
      <ul>
        <li>
          <b>FUNCTION F = RHS ( NODE_NUM, NODE_XY, TIME )</b> evaluates the
          right hand side forcing term F(x,y,t);
        </li>
        <li>
          <b>FUNCTION K = K_COEF ( NODE_NUM, NODE_XY, TIME )</b> evaluates K(x,y,t);
        </li>
        <li>
          <b>FUNCTION G = DIRICHLET_CONDITION ( NODE_NUM, NODE_XY, TIME )</b>
          evaluates G(x,y,t) for nodes on 