Prevent re-indexing when index already exists

[nathancday]

Thinking to add a rebuild = false or force = false argument to the index functions, that would skip indexing when an identically named index already exits. Would save time from accidental reindexing moving between notebooks. A user could still trigger a fresh rebuild when they wanted to and they would be messaged about the skipping behavior.

[nathancday]

Index progress could be linked to #51