Read the binary format from Python

[zrho]

The Python bindings can produce the model-based binary format from the hugr Python class hiearchy, but it can not yet read it back in. The Rust-Python bindings already allow to read the binary format into the Python model types; it therefore remains to implement an "import" process similar to that implemented in Rust.

The closer the core and model formats become (#2291), the simpler and more complete this process can be.

[cqc-alec]

A temporary solution is implemented in #2581 . This should be replaced with a proper implementation that does not go via JSON.