implement option for substructure_preserver in generated Examples space

[ChrisCollison]

I want to be able to have a space of examples for which I preserve a key central substructure. Stoned documentation suggests code for generating that Examples set, by excluding perturbed molecules where the substructure has been modified.
(See https://github.com/aspuru-guzik-group/stoned-selfies/blob/main/stoned_selfies_tut.ipynb)

[whitead]

Not sure how efficient it would be, but you can just filter your subspace if you can define the substructure some how.