Add architecture version when checkpointing

[PicoCentauri]

When loading a checkpoint from an older version of metatrain using a architecture's load_checkpoint and the architecture changes one may gets a very cryptic error message like

RuntimeError: Error(s) in loading state_dict for SoapBpnn:
	Missing key(s) in state_dict: "composition_weights". 
	Unexpected key(s) in state_dict: "composition_model.weights".

To improve this we should add an architecture version inside the checkpoint and check this against the current architecture version. If they disagree we can raise a better error message. The version could be just a simple integer.