Split main and add test

[NicoRenaud]

Hey @kaminotesf I've implemented the suggestions I've made my emails:

1. I've split main.py into two files, one for the tree/forest and one for the sampling of the circuits.
2. I've started some basic test

I'll let you know as soon as the PR is ready for review !

[NicoRenaud]

@kaminotesf I think it should be good enough for a first review. Just a few additions on top of what I've already mentioned:

• I've removed a few unused imports and fixed a couple typehints
• I've slightly modified get_samples() so that it can accepts Aer V1 and runtime V2 samplers. If we want to accept more samplers we should find a more elegant solution
• The tests simply check that the functions run. If we want to compare the output to predetermined results we still have to do that.
• I've modified the CI so that the tests are automatically ran. If we want to link to coveralls (https://coveralls.io/) someone on your side needs to allow coveralls to have access to the repo. This can be done for free once the repo is open. We can then add a badge to the repo to show the coverage.
• I didn't run the notebooks but I think they should still work

[sbryngelson]

@NicoRenaud added coveralls and badge