Merge branch 'issue/doxygen_documentation' into 'develop'

improved doxygen documentation

See merge request !92
4 jobs for develop in 40 minutes and 15 seconds (queued for 30 minutes and 14 seconds)
Status Job ID Name Coverage
  Test
passed #8617
debian:10 clang-6-libcpp-17

00:24:45

passed #8616
debian:10 gcc-8-17

00:18:43

passed #8618
debian:9 gcc-6-14

00:19:20

passed #8619
ubuntu:18.04 clang-6-17

00:21:29