[TASK]: Stub model stratification API endpoint

[bencodrington]

Description
Create an endpoint to solidify the proposed API for stratifying two models in Petri Net form.
Should not rely on any integration with Catlab or TA2 performers.

The input payload should include:

• modelA: a JSON-serialized petri net
• modelB: a JSON-serialized petri net
• typing information
• potentially other inputs

This proposed list of inputs will become more fleshed out as we make progress on #6.

Behaviour and return value
Unlike the equivalent ticket for composition, stratification is non-trivial to implement, so the stub will be more barebones.

For now, simply return a hardcoded JSON-serialized petri net, created by stratifying two simple models (like SIR and Q-NotQ) by hand.