Add elastic workflow for FHI-aims

[tpurcell90]

Summary

1. Create the workflow for FHI-aims
2. Add a new property for sign of the stress tensor. VASP uses the opposite sign
   from FHI-aims

Additional dependencies introduced (if any)

None

TODO (if any)

None

Checklist

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[tpurcell90]

Are any changes needed for this PR?

[tpurcell90]

ready to merge?

[JaGeo]

As this is a larger pull request, @utf needs to check and decide.

[utf]

This looks great, thanks @tpurcell90.