Add a unique method to stats

[nwlandry]

It would be nice to be able to call

import xgi
H = xgi.load_xgi_data("email-enron")
H.edges.size.unique()

[nwlandry]

@apurvabanka --- I saw that you had posted a comment saying that you would like to address this. PR #603 addresses this, but can you take a look at the PR and see if that was what you were picturing? I would really appreciate it!

Also, feel free to address one of our other issues!