TensorGridFactory: return `unique_ptr` instead of `shared_ptr`

StructuredGridFactory switched from returning a `shared_ptr` to
returing a `unique_ptr` in f96b091d.
This commit implements the same change for TensorGridFactory.
5 jobs for feature/tensorgridfactory-return-unique_ptr in 57 minutes and 25 seconds (queued for 81 minutes and 57 seconds)
Status Job ID Name Coverage
  Test
passed #23411
duneci
debian:10 gcc:c++17

00:30:23

passed #23413
duneci
debian:9--clang

00:28:15

passed #23412
duneci
debian:9--gcc

00:26:40

passed #23415
duneci
ubuntu:16.04--clang

00:26:55

passed #23414
duneci
ubuntu:16.04--gcc

00:24:07