Improvement on molecule standardization steps

## Changelogs - Refactored `_standardize_mol` by simplifying the removal of salt, metal etc, standardized for both tautomeric and protomeric forms. - Fix the activity cliff detection and exclude irrelevant rows. - Minor changes on syntax and chemical space visualization. --- _Checklist:_ - [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)._ --- This PR is for updating the standardization steps in molecule curation steps.