Fix global_dof_index in matrix-free class: It should only be used when dealing with...

Fix global_dof_index in matrix-free class: It should only be used when dealing with deal.II's global dof indices, not the matrix-free dof indices which are transformed into a process-local index space and start at zero on all processors. This limits the MPI-local problem size to 4 billion, but i) that limit is still far away and ii) there were many internal uses of unsigned int left that would have needed re-organization. This change saves memory as dof indices can be saved in 32-bit integer instead of 64-bit integers and still allows problems with >4g global dofs.

git-svn-id: https://svn.dealii.org/trunk@30090 0785d39b-7218-0410-832d-ea1e28bc413d