Add versioning information to all model files

[joanise]

Description & Motivation

Vocoder Checkpoints, Feature Prediction checkpoints, etc, need to have versioning information, for two purposes:

• simplify error messages when the wrong type of model gets loaded
• recognize that a file is in the format of an older version, and support converting it on the fly if possible or giving good error messages if not.

Pitch

Just like .readalong files now have file format version numbers, we really need this for EV models too.

[joanise]

And let's make these fields Optional so that we don't invalidate our existing checkpoints: let them default to the first version number if not found, but let's henceforth generate them every time we create model files & checkpoints & config files.