move keys from sigmf.SigMFFile to top-level sigmf

[Teque5]

It's bothered me ever since the beginning that if you want to set or get some key within sigmf you needed to do sigmf.SigMFFile.VERSION_KEY instead of just doing sigmf.VERSION_KEY. Now we appear to also have keys within archive.py for file extensions, which is strange.

Other modules (logging, pygame, numpy) have global vars like this in the top level.

I would like to:

1. Consolidate all keys into keys.py or similar name.
2. In the __init__.py do from .keys import * so that we can access a key using sigmf.VERSION_KEY.
3. In SigMFFile attach keys as properties for now and insert a comment saying that functionality will be deprecated in a future version. This keeps backwards compatibility.

This change should increment the minor version (major.minor.patch).