Refactor code according to upstream changes

[tdoublep]

This PR reworks our code according to some important upstream changes. In particular, there is no longer any need to have a separate SpyreExecutor and MultiprocessingSpyreExecutor`. Upstream has added generic classes for this that work across different platforms. Acutally, it simplifies our code quite a lot.

TODO - the embeddings stuff needs to be re-worked a bit too

[github-actions]

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[tdoublep]

Still needs some work; moved back to draft.