Change the type of inverter bus to 4 instead of 2 in the math model

[haoxiangyang89]

The matrix construction relies on identifying the inverter buses. The inverter buses have a different bus_type under the math model.

Working with engineering model might be another option but requires more changes in the matrix construction processes.

[haoxiangyang89]

Changing bus_type will lead to errors related to solving process. Instead of changing the current data structure, we added new data fields to the data structure.