Graded Homework 3

As always, great work, Jenny!

A few comments I have are:
- You don't need to type cast unnecessarily. When in doubt, always try running `type(...)` of whatever you have questions about!
- Huge kudos to you for realizing that you should raise a flag when a user types in something that's not one of the nucleotides! Search for 'python raise exception' for more information on these, or feel free to check out the codes in [my hw3 file] (https://github.com/abekim/softdes/blob/master/homework/hw3/gene_finder.py)