Molecular transformer dataframe input

[thibaultvarin-r]

Changelogs

• enumerate the changes of that PR.
  In MoleculeTransformer.transform,
• If mols is a dataframe, then the first column of this dataframe is saved in mols as a pd.Series.
• Also features generated are passed to utils.check_array from scikit-learn and then returned

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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 new introduced feature(s) (if appropriate).
• Update the API documentation if a new function is added, or an existing one is deleted. Eventually consider making a new tutorial for new features.
• Write concise and explanatory changelogs below.
• 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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discussion related to that PR
It was discussed first with @maclandrol and Ben Fogelson.