Add keyboard interrupts for subgraph.find_subgraph

[boothby]

I've finally figured out a (hopefully) good-enough means to halt on ctrl-c within find_subgraph. Sadly, testing this capability in CI has proven flaky in the past (as seen in tests/test_lib.py:841 ). I'll try and scrounge some folks on windows and osx to do some tests.

[boothby]

Test procedure:

1. run the following code, once in a repl (python, ipython, or your favorite IDE) and once as a script (from the commandline: python test.py where test.py has the code below)

from minorminer import subgraph, find_embedding
from time import sleep
import dwave_networkx as dnx
source = dnx.chimera_graph(8)
target = dnx.chimera_graph(15, coordinates=True)
target.remove_node((7,7,0,0))

emb = subgraph.find_subgraph(source, target)
print("caught an interrupt in find_subgraph")

emb = find_embedding(source, target, chainlength_patience = 1000000, interactive=True, tries=1)
print("caught an interrupt in find_embedding")

emb = subgraph.find_subgraph(source, target)
print("caught an interrupt in find_subgraph")

try:
    sleep(1000)
except KeyboardInterrupt:
    print("caught an interrupt in sleep")

2. hit ctrl-c four times, waiting until you see some output before pressing it again
3. the output should look something like

^Ccaught an interrupt in find_subgraph
^Ccaught an interrupt in find_embedding
^Ccaught an interrupt in find_subgraph
^Ccaught an interrupt in sleep