Trying to fix series.py

Hey Chris, 

This pull request was intended to try and fix the issue you mentioned for series.py where I wasn't taking a user value for the final series. I think I've solved the issue, but wanted to get your thoughts on it. 

In addition, I see several deletions in this pull request that I don't see deleted on my local repo, or my fork (and I didn't mean to delete many of these files, perhaps it just thinks I'm moving them?)

Thanks for your help, 

Schuyler
.isdigit requires a string for some reason (I think because it's a string method)