Split Branch into Line and Transformer

[ccoffrin]

This will be more consistent the distribution mathematical model and will improve the accuracy of the model used for transformers.

[ccoffrin]

This will also simplify the implementation of OLTC.

[ccoffrin]

Revisit tr-tp-models branch once this is done.

[ccoffrin]

Consider addressing #575 here as well.

[ccoffrin]

This will also be a good time to revisit the naming of constraints like,

constraint_ohms_yt_from(pm, i)
constraint_voltage_angle_difference(pm, i)
constraint_thermal_limit_from(pm, i)

Should probably have names like constraint_line_power_flow_from, constraint_line_thermal_limit_from.