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?
  - [ ] Limit SUNDIALS to 6.7.0 as per bout docs? (Seems to build fine with 7.0, though)
  - [ ] 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

Leaving PETSc and SUNDIALS versions unrestricted for now.
coverage variant 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