Functionalize and document create_cyclus_input.py

[nsryan2]

Currently, scripts/create_cyclus_input.py is unlike the other scripts in that it's actually like a script. This issue is mostly concerned with finding a way to document it (with examples where possible). If it can be turned into a series of functions like the other scripts, that would be a great addition.

This issue can be closed when there is a way to document scripts/create_cyclus_input.py with an example.

[RhysMacMillan]

One spot I couldn't close in a PR regarding start year assumptions was in this script. It is assumed that start_year = 1965 as a global variable, along with some other global variables in the beginning. I thought about having a user input for the start year with a default of 1965 to satisfy current tests, but I decided to mention it here and not change it.