createBH Creates a boundary topology

 USAGE:

       [BH, BMother] = createBH(H, HN);

       [BH, BMother] = createBH(H, createHN(H));


 DESCRIPTION:

       Given topology H and its neighbourhood HN, will create
       boundary topology BH.  Can also produce BMother which defines
       index into H to which boundary element belongs to.


 INPUT:

       H           - Topology matrix
       HN          - Neighborhood topology matrix

 OUTPUT:

       BH          - Boundary topology matrix
       BMother     - Vector in which each row defines an index in H to which the
                     boundary element belongs to

 This function is provided with ValoMC