Updated PyDASSH with passing unit test

Hi all,

I realized the implemented logic for handling inheritance and assignment of subassembly parameters was not executing properly. I implemented new logic for the assembly parameter assignment that works to pass the simple, preliminary unit test I developed. I also cleaned up PyDASSH.py to improve code correctness, readability, and style. (As an aside, Ahmed, I would highly recommend finding a "linter" to develop with - it checks your code as you write it to call out potential errors.)

I have a larger input test I'll be implementing tomorrow, and then I'll start on some of the tests that should purposefully fail to catch errors. Looking forward to your comments.

-Milos