graph compatibility for generalized spatial indices

[knaaptime]

the Graph implementation of distance band requires point geometries (unlike the W version). So any spatial index that calls Graph.build_distance_band needs to be updated to either 1) do the casting internally, or 2) raise with a more informative message

I think casting is probably ok, but technically it would be better to force users to reckon with the downcasting themselves (e.g. do you want centroid or representative point?)

[knaaptime]

this actually opens a can of worms for the batch_compute functions because many of the spatially-explicit indices like PAR dissimilarity or concentration require the polygon representation