Migrating to glasflow

[mj-will]

Long term, we plan to move away from the flows defined internally in nessai and directly use those from glasflow.

Requirements

• Support for using different neural networks (ResNet vs MLP) in glasflow
• Decide how to handle missing methods finalise, end_iteration, freeze_transforms and unfreeze_transforms

[mj-will]

The first step to this was adding experimental support for using glasflow flows directly, see #386