Add basic manifest and CI

[CodeGat]

Closes #47

Background

Here is an initial implementation of build-ci for this repository. It contains a simple install test, found in .github/build-ci/manifests/spack.yaml.j2. More can be added, and more can be done in parallel, with either a matrix or multiple jobs. `

[aidanheerdegen]

My preference, if this is suitable, is to merge and add more bells and whistles later if required.

Not only does that get it deployed more quickly, but it also serves as a minimal example that others can just copy/cherry-pick.

[CodeGat]

https://github.com/ACCESS-NRI/mppnccombine-fast/actions/runs/16557063923/job/46820064243?pr=50 2 minute CI 🥹

[CodeGat]

@dougiesquire is this a kind of package that would benefit from having all the same compiler down the dependency chain, or does it not matter as much?

[dougiesquire]

I'm not sure sorry. I don't really understand when this is/isn't a good idea?

My guess is that it's less important here, so maybe we start without and see how we go?

[dougiesquire]

Thanks @CodeGat!

Code you please also remove the .travis.yml file and remove this line from the README.md? Then I think this PR can close #47

[CodeGat]

Done in 56b1154 @dougiesquire

[dougiesquire]

Thanks @CodeGat - LGTM