Revision 1669

A problem with VTK writer and MPI is fixed: Apparently MPI_file_open does not "delete" the file  which has the effect that 
if less date are written into the file as the file contained when opened bits of the previous containt remains in the file. This problem is fixed
by deleting the file if it exists before open it with MPI. The additional function Paso_existFile needed to be added as there is no standart C 
function to test the exists of a file.