Implement bus <--> signal node connections in phasor dynamics models

Summary

In order to allow for GridKit to be used in co-simulation context, implement connections allowing for bus voltages to be read by a signal node. Similarly, implement connections where buses can read injection currents from a signal node.

Rationale

Co-simulations are commonly used in industry to connect different simulation tools or to include hardware data streams in hardware-in-the-loop simulations. Phasor domain dynamic co-simulated models are connected by exchanging bus voltages and injection currents between co-simulated parts. Providing signal connection to buses is a step towards enabling co-simulations in GridKit.

Description

Consider adding signal ports like this to the bus model:

• voltage_r
• voltage_i
• injection_current_r
• injection_current_i
  Voltage ports should be outgoing , while current ports should be incoming signal ports.

This is a good time to review signal ports implementation.

Additional information

CC @abirchfield @lukelowry @alexander-novo