Make TreePath<i...> an alias for HybridTreePath<index_constant<i>...>

5 jobs for feature/merge-treepath-classes in 2 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #143056
duneci
debian:10 clang-7-libcpp-17

00:01:48

passed #143055
duneci
debian:10 gcc-7-17

00:01:52

passed #143054
duneci
debian:10 gcc-8-noassert-17

00:01:50

passed #143053
duneci
debian:11 gcc-9-20

00:01:51

passed #143057
duneci
ubuntu:18.04 clang-6-17

00:02:40