allow for time- and spatial dependent Dirichlet boundary condition

Hi,
   this is an extension of the Dirichlet boundary condition allowing the boundary value to be a function of both time and the spatial variables. Please Klaus can you check, if we can add this to the fd framework. Actually I thought it was already commited, but I can not find it in the current repository. I will add tests later. The current test suite works fine with the change.
regards
   Peter