Added a Scikit-learn like `train_test_split` method

Changelogs

Checklist:

[X] Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
[X] Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
[x] Update the API documentation if a new function is added, or an existing one is deleted.
[X] Write concise and explanatory changelogs above.
[X] If possible, assign one of the following labels to the PR: feature, fix or test (or ask a maintainer to do it for you).

You can now do:

from splito import train_test_split

X_train, X_test, y_train, y_test = train_test_split(X, y, smiles=smiles, method="scaffold")