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
• i) Delete spack-generated build links to keep submodule clean OR ii) ignore untracked in BOUT-dev OR iii) Make a PR to modify BOUT-dev/.gitignore (went for option i)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.41%. Comparing base (e0f1ebf) to head (29aaea1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #376   +/-   ##
=======================================
  Coverage   26.41%   26.41%           
=======================================
  Files          90       90           
  Lines        8105     8105           
  Branches     1133     1134    +1     
=======================================
  Hits         2141     2141           
  Misses       5745     5745           
  Partials      219      219           

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[oparry-ukaea]

N.B. Ready to review, but merge should probably wait until other BOUT-spack changes have gone in to avoid confusion.