cmake: FetchContent for Metis

Following on from the discussion in issue #1018 (closed)

Aim

Remove dependence on the internally-packaged metis for min-builds of Octopus with cmake.

Issues

@LecrisUT wrote the branch feat/update-metis, which introduces fetchContent for Metis. However, the original upstream has no tags and Crisitian's fork of HEAD (version 5.2.x) produces a segfault when run with Octopus. Octopus's API is coded for Metis 5.1.0.

Solutions

a) One may be able to repoint Cristian's branch to this fork (which is consistent with what the linux distros are starting to do) for version 5.1.0. This would allow the removal of the internal version.

b) Optionally, once fetchContent works, point to HEAD of Cristian's fork or Scivision's fork, and fix the Octopus API for Metis 5.2.x