Resolve "Users adding properties to classes that aren't used"
One feature of Python can cause a lot of trouble for users of this library. Classes defined in this library have very specific values defined for each, values that are printed to a file prior to DIRSIG running. Most of the time, users use the setters/getters to interact with those values, but on occasion, they try to access them directly. If the user includes a typo or somehow unintentionally creates a new variable in that class, the desired effect they want to generate will be hidden away because that new variable will never get used.
This pervasive base class makes it impossible to generate new class properties after __init__ is called.
Also updated CONTRIBUTION.md.