`ProblemDefinition` can’t read old disk format 😢

[xroynard]

https://github.com/PLAID-lib/plaid/blob/main/src/plaid/problem_definition.py#L1220-L1225

it tries to access keys input_features and output_features which are present only in the last version of the disk format

#14 was not checked enough

we should improve tests