Add int64 variant to package.py

[ebchin]

Summary

• This PR is a small addition to the axom spack recipe.
• It does the following:
  • Enables setting AXOM_USE_64BIT_INDEXTYPE through the spack variant int64

Context

Recently, we've been running large problems in Tribol which have needed this option. Exposing it to the spack recipe will simplify enabling this in our TPL builds.

[kennyweiss]

Thanks for adding this variant @ebchin

I wonder if there's a better name we could use for this though (but nothing comes immediately to mind). I checked the hypre package, and they use int64 for an equivalent option, so I suppose we should go with this.

See: https://github.com/spack/spack/blob/f0a7388496a5cb062d7ec6541273b6cc697ba051/var/spack/repos/builtin/packages/hypre/package.py#L65

[kennyweiss]

Side question for axom team -- we previously discussed setting the default IndexType to 64-bit.

Would this be a good time to do this? E.g. by making the default spack variant +int64?
See: #718

[ebchin]

Yeah, the int64 name came from hypre. Happy to change if there's a consensus on something better.

[rhornung67]

@kennyweiss I think changing the default index type in Axom to 64-bit int is a good idea. It would probably prevent some user issues and others can set it to a lower resolution type if they want/need to.

[kennyweiss]

Thanks @rhornung67.

In that case, @ebchin -- could you please change the default for int64 to True?

(we'll change the default CMake option on the axom side in a follow-up PR).