(Undirected) Edge indexing mode

[isaacbrodsky]

#133

[coveralls]

Coverage: 98.766% (+0.1%) from 98.657% when pulling 7aad06bb798821f12eaafd9b4d5f10da4d8bfd7a on isaacbrodsky:edge into 5c91149104ac02c4f06faa4fc557e69cf6b131ef on uber:master.

[isaacbrodsky]

This PR is now ready for review

[ajfriend]

I think it makes sense to keep the API agnostic as to the ordering of the cells, but the internal representation should always have them in the "canonical ordering". Does that seem like the right approach?

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

:white_check_mark: isaacbrodsky
:x: Isaac Brodsky

---

Isaac Brodsky seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}