make ParameterTree capable of having internal references

9 jobs from feature/references_in_parametertree in 16 minutes and 26 seconds (queued for 8 seconds)
latest
Status Job ID Name Coverage
  Test
passed #67337
duneci
debian:10 clang-6-libcpp-17

00:03:12

passed #67335
duneci iset:avx allowed to fail
debian:10 gcc-7-14--expensive

00:11:00

passed #67336
duneci
debian:10 gcc-8-noassert-17

00:06:08

passed #67339
duneci
debian:9 clang-3.8-14

00:02:07

passed #67772
duneci
debian:9 gcc-6-14

00:05:25

passed #67773
duneci
ubuntu:16.04 gcc-5-14

00:04:57

passed #67341
duneci
ubuntu:18.04 clang-6-17

00:02:34

failed #67338
duneci
debian:9 gcc-6-14

00:00:41

failed #67340
duneci
ubuntu:16.04 gcc-5-14

00:00:40