Merge pull request #4230 from masterleinad/cxx17_new

Use C++17's [[fallthrough]]

diff --cc include/deal.II/numerics/vector_tools.templates.h
Simple merge

diff --cc source/base/data_out_base.cc
Simple merge

diff --cc source/fe/fe_q_base.cc
Simple merge

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

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