Options when dropping duplicate geometries

[tomalrussell]

Use case: retaining data about nodes or edges which may share geometries.

For example, a set of nodes at the same point location with different names/ids which should be retained or combined.

Enable something like (pseudocode):

nodes = [
    combine(cluster)
    for cluster in network.nodes.groupby(location):
]

Also, warn when dropping duplicate geometries within other snkit functions.