This adds Identifier, IdentifierOption, PureRecord, and SegmentRecord to feos.ideal_gas.

In the short term this allows to handle ideal gas parameters like other models. For the future, there remain 3 issues:

1. In Python, we have a PureRecord for every model, which can lead to confusing errors or the requirement to rename imports.
2. The PureRecord in feos.ideal_gas refers to Joback specifically, so if we add more models, the naming will be awkward.
3. It might be more clear if non-generic Python classes (Identifier and IdentifierOption) are exported only once.