Transformers

[maclandrol]

Changelogs

• Make transformers optional

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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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This PR makes the transformers library optional, so that importing molfeat module will not fail. All other module and submodule will work as previous.