Update the spack config to build BOUT++ as a separate package

[oparry-ukaea]

Updates the BOUT-spack submodule to make BOUT++ a separate spack package from Hermes-3 Also makes it a 'develop' package in spack.yaml.

Related to boutproject/BOUT-Spack#6 .

ToDo:

• [ ] Update spack instructions in the docs
• [ ] Update activate_h3env script to manage build links for BOUT
• [ ] Delete spack-generated build links to keep submodule clean OR ignore untracked in BOUT-dev OR Make a PR to modify BOUT-dev/.gitignore