Deprecate ToUniquePtr.

It was just introduced to allow to  show a
deprecation warning in case someone consumes a
GridFactory::createGrid() into a raw-pointer.
12 jobs for feature/deprecate-to_unique_ptr in 8 minutes and 28 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #197687
duneci
debian-11-gcc-9-17-python

00:04:35

passed #197678
duneci
debian:10 clang-7-libcpp-17

00:01:45

passed #197676
duneci
debian:10 gcc-7-17

00:02:24

passed #197686
iset:avx duneci
debian:10 gcc-7-17--expensive

00:08:22

passed #197677
duneci
debian:10 gcc-8-noassert-17

00:02:45

passed #197680
duneci
debian:11 gcc-10-20

00:05:43

passed #197679
duneci
debian:11 gcc-9-20

00:02:36

passed #197681
duneci
ubuntu:18.04 clang-5-17

00:02:41

passed #197682
duneci
ubuntu:18.04 clang-6-17

00:03:04

passed #197685
duneci
ubuntu:20.04 clang-10-20

00:02:51

passed #197684
duneci
ubuntu:20.04 gcc-10-20

00:04:42

passed #197683
duneci
ubuntu:20.04 gcc-9-20

00:04:07