Share i/o descriptions with model bundle

[phildow]

Finding that we often need access to the inputs and outputs descriptions without the model itself. Move those descriptions to the model bundle, parse there, and reference them from the model itself.

I'm thinking we use a TensorIOInputsOutputs class that encompasses the inputs and outputs and places them on an io property on the model bundle and on the model, so you have:

bundle.io.inputs
bundle.io.outputs

And

model.io.inputs
model.io.outputs