Add ZSNet

[mmuckley]

This PR will implement code for reproducing results in the following paper:

Radmanesh, A., Muckley, M. J., Murrell, T., Lindsey, E., Sriram, A., Knoll, F., ... & Lui, Y. W. (2022). Exploring the Acceleration Limits of Deep Learning VarNet-based Two-dimensional Brain MRI. Radiology: Artificial Intelligence, e210313.