Need better compilation process & documentation

[mikekryjak]

The "default" Hermes-3 compilation procedure loads BOUT++ as an external module, but it does not download and compile PETSc, and no guidance on this is provided in the documentation. I think we need to implement scripts to get PETSc in the correct way (as per @johnomotani's scripts and my version of them). This is especially important since the example cases are now using PETSc solvers by default.

• [ ] Add guidance to documentation, including the required versions of BOUT++ and PETSc and references to install scripts
• [ ] Implement PETSc download and compilation in CMake (?)

@bendudson @johnomotani is the latter possible?

[bendudson]

I think the easiest way to accomplish this is with a Spack package. PETSc has a package, including MPI and hypre variants. We could perhaps add hermes-3 to the official package repo, but for testing I've made our own BOUT++ repo: https://github.com/boutproject/BOUT-spack More documentation needed of course...

[mikekryjak]

Another user just spent quite a bit of time being confused about how to install PETSc with Hypre capability. I think we should update this page in the docs (https://hermes3.readthedocs.io/en/latest/getting_started.html#building-with-petsc) with:

• A guide on how to download and compile PETSc, possibly with this code snippet: https://github.com/mikekryjak/bout-build-scripts/blob/086fea8c7bdb28d66b72508a3251e64d0929ffa8/1-build-dependencies.sh#L36-L43
• A guide on how to link Hermes-3 to an existing BOUT++ installation
• A guide on the basics of compiling (tips for beginners, e.g. if you want to build another branch, you can compile it into a different folder, and you don't need to remove everything to recompile)