Merge pull request #12953 from bangerth/unsigned

Make dimension counters unsigned.

diff --cc include/deal.II/matrix_free/cuda_matrix_free.templates.h
Simple merge

diff --cc include/deal.II/matrix_free/shape_info.templates.h
Simple merge

diff --cc source/distributed/tria_base.cc
Simple merge

diff --cc source/dofs/dof_tools_constraints.cc
Simple merge

diff --cc source/grid/grid_tools.cc
Simple merge

diff --cc source/grid/grid_tools_dof_handlers.cc
Simple merge