Merge pull request #16954 from bangerth/tolerance

Consistently use a relative solver tolerance.

diff --cc examples/step-16/step-16.cc
Simple merge

diff --cc examples/step-4/step-4.cc
Simple merge

diff --cc examples/step-40/step-40.cc
Simple merge

diff --cc examples/step-5/step-5.cc
Simple merge

diff --cc examples/step-6/step-6.cc
Simple merge

diff --cc examples/step-7/step-7.cc
Simple merge

diff --cc examples/step-8/step-8.cc
Simple merge