Added functions to DBM class to solve Laplace's eqn.

UpdatePhi is a controlling function which calls SolveLaplace and
TestConvergence iteratively until the solution has converged to
within the desired tolerance as set by CnvgCrit.

TestConvergence finds the maximum difference between old and new
solutions for Phi and returns true if this value is less than the
required tolerance as set by CnvgCrit.

SolveLaplace solves Laplace's eqn. using a finite difference scheme
either using a Jacobian or Gauss-Seidel type update procedure.