topk

[rogerpearce]

After #56 is merged. Create a PR off master to do this.

Add a topk method that takes as arguments:

• Required: node or edge series to top-k
• Required: k
• Optional: Where
• Optional: meta (additional node/edge series to include in output, must match node/edge of top-k series)
• Optional: ascending = True

Returns array of k entries

[sbromberger]

As discussed - ascending=True goes away - we want a separate bottom-k function instead.

[sbromberger]

Closed via #72