Make BOUT++ a separate package.

[oparry-ukaea]

Adds a package file for BOUT++ and makes it a dependency of the Hermes-3 package.

Resolves #3.
Resolves #9.

ToDo:

• Remove adios, netcdf (?), petsc, sundials dependencies from hermes-3 (set in boutpp directly)
• Set the right dependencies for boutpp+python
• Set maintainer(s) for boutpp
• Bout updates to avoid CMakeLists patch
• Check variant defaults match BOUT CMake defaults.
• Limit PETSc to 3.23 as per bout docs (boutpp@5.0.0 further restricted to PETSc <= 3.17)
• Limit SUNDIALS to 6.7.0 as per bout docs (Seems to build fine with 7.0, though!)
• Remove pvode variant
  - [ ] Add "coverage" variant (conflicts if compiler isn't gcc or clang); defaults to off
• Disable strumpack variant for now?? numpy build fails; looks like this issue

Skipping coverage variant, for now; can be added later if necessary.

[oparry-ukaea]

N.B. fa6fefc can be reverted if using a version of bout with the changes from boutproject/BOUT-dev#3101

[mikekryjak]

Great work on this @oparry-ukaea, it really seems like you've managed to capture almost all of the default behaviour properly (at least as far as I can tell). I checked by running a simulation with this version and with my existing installation and comparing the BOUT++ console dump. I am yet to compare the configure console output.

[mikekryjak]

Both SUNDIALS and PETSc need to be fixed to the latest version in the CMake. Upgrading these versions is not always trivial.

[oparry-ukaea]

SUNDIALS now limited to versions between 2.6 and 6.7.0 as per the bout docs.
PETSc now limited to 3.7:3.23 as per the bout (5.2.0) docs (I also needed to restrict boutpp@5.0.0 to PETSc <= 3.17 to make it build).

[ZedThree]

I think SUNDIALS 7 and PETSc 3.24 do work, although that will depend on the BOUT++ version

[oparry-ukaea]

Happy to change it if you're reasonably confident; it certainly seems to build fine with sundials@7.0.0, but I haven't run the tests.

[oparry-ukaea]

Think this is ready to merge.