spatial input

[mpadge]

Needs to accept both sf and sp

[mpadge]

Nah, that just raises questions of what the objects would be that were being clustered? What if linestrings were submitted - would clustering then be performed based on the linestrings? Or based on the points of which they are composed? Better to leave it to users to first extract representative and simple coordinates for each object to be clustered, and submit those.