[bugfix] Fix member functions returning iterators

Member functions returning iterators need to be overwritten
in the derived interator class. Otherwise they would return
base class iterators.

Here we duplicate the implementations. This could be avoided
using CRTP and/or mixins. However, this would lead to more
complicated and even longer code.
7 jobs from feature/add-transformedrangeview in 7 minutes (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #77135
duneci
debian:10 clang-7-libcpp-17

00:02:34

passed #77140
duneci iset:avx allowed to fail
debian:10 gcc-7-14--expensive

00:06:59

passed #77134
duneci
debian:10 gcc-8-noassert-17

00:03:35

passed #77137
duneci
debian:9 clang-3.8-14

00:02:17

passed #77136
duneci
debian:9 gcc-6-14

00:03:15

passed #77138
duneci
ubuntu:16.04 gcc-5-14

00:02:41

passed #77139
duneci
ubuntu:18.04 clang-6-17

00:02:55