ADD: PowerModelsDistribution version of correct_bus_types

[pseudocubic]

Currently, in the engineering model bus_type is inferred from both the fields on the component, e.g. if vm and va is present then the bus would be assumed to be a reference bus, and from the components connected to the bus, e.g. if a voltage_source is connected then the bus would be a bus_type=3 in the mathematical model, and bus_type = 2 if a PV generator is attached to said bus. However, if the generator is droop we might expect that the bus_type != 2 as correct_bus_types current does.

Related to change made in PR #295

[pseudocubic]

fixed in v0.11 releases