ENH: Initialize Aspire object with existing flow

[jacopok]

Addresses #20.

It's quite simplistic, but I cannot see what else would be required. I can rebase after #21 is merged if that's preferred.

I added a test, probably being overzealous; I can remove it.

[mj-will]

Thanks @jacopok, this looks good to me. I'm happy to get this in before #21

It seems the linting is failing, it's probably worth using the pre-commits to fix this since that will ensure the version used locally matches the CI.

[jacopok]

I see, thanks! I will definitely set up the pre-commit hooks for the future; for the moment, I think the latest commit I made should be enough (I had manually linted the package source only, forgetting to do the same for the tests).

The test I added is now in the integration section of the CI - it may be worth moving to a new "unit test" section to be only run once in the future.

[mj-will]

Yeah, cleaning up the tests is on the list of things to do but for now I'm happy to keep this as is for now.