Added a Scikit-learn like train_test_split method

[cwognum]

Changelogs

• Closes Make a general function for all splitting strategies. #8 by adding a general train_test_split method

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Checklist:

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

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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")