Organize `data` and python modules by namespace

[ahouseholder]

This is intended to be dealt after #749 is merged.

We currently have things in the decision point directories in both the data and python folders organized where the "ssvc" items are in the main dir, and the "cvss" ones are in a subdirectory. However, because we're thinking of these in terms of namespaces, it probably makes more sense for us to move the ssvc things into an ssvc directory parallel to the respective cvss directories.

Tip

While this might look like a trivial mkdir ; mv operation, there will be implications to python imports, unit tests, markdown docs, etc., that will need to be resolved. I highly recommend doing this with the help of an IDE that supports refactoring of this type. That will make it a lot easier than fixing everything by hand.