Use DofMap.local_to_global_index() to get global dof index in DofHandler._get_changes

[francesco-ballarin]

Dear fenicstools devolpers, if I understand correctly DofMap::cell_dofs returns local dof indices, and not global ones. If that is really the case, then DofMap::local_to_global_index should be used in DofHandler._get_changes to get the global dof index, shouldn't it?

Please let me know if I am correct. If not, I will close this PR.

Thanks, best regards, Francesco

[MiroK]

Thanks Francesco.