Support method to combine overlapping polygons in a Graph

[TaipanRex]

overlaps = True
while overlaps:
overlaps = False
for each polygon n:
for each polygon m:
# https://techoverflow.net/2017/02/23/computing-bounding-box-for-a-list-of-coordinates-in-python/
# checking bounding boxes is a quick way to see if two polygons potentially overlap.
if the bounding box of n overlaps with the bounding box of m:
# combine the polygons or return both if they don't actually overlap
https://stackoverflow.com/questions/2667748/how-do-i-combine-complex-polygons
overlaps = True

Worst case this runs in O(n^3), i.e. all polygons overlap.

[deboc]

Personally, I preprocess the polygons with cascaded_union from shapely.ops.
I guess you want to avoid external dependencies. But I also guess that's the best implementation we can find.
(And shapely could also be useful for other preprocessing, as for #21)

[TaipanRex]

@deboc thanks for the suggestion. Yeah, I have tried to avoid dependencies. I am working on proper documentation on branch documentation. I might just reference Shapely and the specific function with an example, then let it be up to the user to do it.