Merge branch 'issue/make_datatransferoperation_enum_class' into 'develop'

make DataTransferOperation an enum class

See merge request !99
4 jobs for develop in 41 minutes and 4 seconds (queued for 7 minutes and 16 seconds)
Status Job ID Name Coverage
  Test
passed #8803
debian:10 clang-6-libcpp-17

00:24:55

passed #8802
debian:10 gcc-8-17

00:17:55

passed #8804
debian:9 gcc-6-14

00:17:57

passed #8805
ubuntu:18.04 clang-6-17

00:23:06