add support for patterns in scaffold decoration

[maclandrol]

Changelogs

• Add support for pattern-based decoration in SAFE.

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Checklist:

• 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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In this PR, scaffold decoration with SAFE is extended to support molecular patterns (SMARTS). Note that the full specification of SMARTS is not supported in this implementation due to various limitations with the tokenizer and how SMARTS patterns are matched with the tokens for constrained-sampling.

[zhu0619]

Thanks @maclandrol

The generation process is the constrained generation based on scaffold with pattern, followed by fragment extension.
Could you also add a few bullet points docstrings on the generation logic for both classes to make the explanation easier to understand for others?

[maclandrol]

Thanks, I will address those comments.

[maclandrol]

All fixed now. Thanks @zhu0619