Expose reset!(mapper) to the user

[Datseris]

Necessary steps for this:

• Make basins_of_attraction or basins_fractions return copies of the stored attractors
• Ensure reset! resets the ID of latest attractor to 1. It doesn't matter for continuation what ID we set to as we do matching anyways at the end.
• Add docstring to reset! and export the function and add it to the docs.
• Define behavior of the function for the featurizing mapper. For proximity mapper it should do nothing.