Allow for Neighborhood to split into identical states

[teald]

It would be useful to spawn new Neighborhoods (and corresponding data) on command.

Motivation and example

I have an extant Neighborhood object, nbr, that I want to stop at some point and then feed two different inputs. Currently, if I wanted to manage this I'd need two instances to get the job done. This is a hassle if any initializations are required that take appreciable time. It would be easier to initialize a single Neighborhood and then have it run the two cases in tandem.

Consider the following example:

# Assume `nbr` was previously initialized as a Neighborhood()
# Run 10 steps
for _ in range(10):
    nbr.run_step()

# Now take this instance and create a new Neighborhood using '.fork()',
# which returns a new Neighborhood object with the same state as nbr.
nbr_other = nbr.fork()

print(nbr_other == nbr)
# Out: "True"

# Now, could run these two as if they were independent neighborhoods.
for _ in range(100:
    nbr.run_step()
    nbr_other.run_step()

# At this point, nbr and nbr_other will have significantly diverged.
print(nbr_other == nbr)
# Out: "False"

With this, more efficient comparison of the two models can be perform as they each evolve.

Implementation specifics

The primary barrier to this being immediately possible is deciding how copying will be applied to the new neighborhood object.