Spack package files and environment config

[oparry-ukaea]

The main aims of this PR are to

1. Make hermes-3 and xhermes easily installable via spack, such that the tests all run out-of-the-box.
2. Facilitate future development of hermes-3 in a spack environment.

Details:

• Adds a spack package for hermes-3, adapted from one in the BOUT-spack repo.
• Adds spack packages for xhermes and those of its Python dependencies that don't already have a published package. All are retrieved via PyPI.
• hermes-3 variants are
  • limiter - turns on HERMES_SLOPE_LIMITER
  • petsc - adds (existing) petsc package as a dependency and turns on BOUT_USE_SUNDIALS (defaults OFF)
  • sundials - adds (existing) sundials package as a dependency and turns on BOUT_USE_SUNDIALS (defaults ON)
  • xhermes - adds py-xhermes package as a dependency (default ON)