Support 0.5 models

[alanocallaghan]

Fairly minimal changes as the updated model spec tries to be transparent, but took the time to replace some deprecated calls.

I was concerned about handling multiple inputs and outputs in BioimageIoTest as we currently allow for , but I realise now that:

• you can already have multiple inputs and outputs in 0.4 and we definitely don't handle those
• if you load an 0.5 model it won't find any of the test inputs and outputs anyway
• we would have to completely refactor how classification is done to handle these anyway! (I think... maybe I underestimate the flexibility of DnnModel etc)

Ideally we would get the test input/output working with 0.5 models with single inputs and outputs too, though...

I've tested with the broad U-net model but would like to ensure it works with some other models and ideally some 0.5 models... See bioimage-io/spec-bioimage-io#683