added documentation of DOFVector and DOFMatrix and improved the documentation of some other classes

Status Job ID Name Coverage
  Test
passed #8513
debian:10 clang-6-libcpp-17

00:31:00

passed #8512
debian:10 gcc-8-17

00:20:36

passed #8514
debian:9 gcc-6-14

00:22:30

passed #8515
ubuntu:18.04 clang-6-17

00:29:19