merge enum-class branch

4 jobs for example/cahn_hilliard in 43 minutes and 38 seconds (queued for 24 minutes and 44 seconds)
Status Job ID Name Coverage
  Test
passed #8809
debian:10 clang-6-libcpp-17

00:24:54

passed #8808
debian:10 gcc-8-17

00:18:03

passed #8810
debian:9 gcc-6-14

00:19:05

passed #8811
ubuntu:18.04 clang-6-17

00:21:56