[FEATURE] threading for multiple models, instead of one model with multiple update_data

[leiwu84]

My use case usually requires say 100 models, i.e. 100 instances of PowerGridModel. At this moment, its batch calculation feature only allows one model with many different update_data. I would suggest allowing batch calculation for many models. Then I can use multiple threads to run power flow for all the models in parallel. For example, I can run all 100 power-flows in one round if I have 100 threads, or five rounds with 20 threads, and so on.