spec_boundary_values function created

I added the spec_boundary_values function, however the content is yet TO
DO, I also modified the main.c to call the function. Write (in
boundary.c) the procedure void spec boundary val (char *problem, int
imax, int jmax, double **U, double **V), where, depending on the
considered problem (given by the variable problem), additional boundary
conditions are implemented. This includes any kind of inflow conditions,
and also special settings, as they are for example present in the driven
cavity example (setting u = 1 at the upper boundary).