Differentiate between signal inlets and outlets in input file format

Since signals are directed connectors, the modeling framework should recognize differences between component's signal inlets and outlets. This should also be reflected in JSON input file format specifications.

For example, current specification for a generator model defines exciter_signal and governor_signal, but for the sake of modeling and creating signal connections, we need to specify

• exciter_signal_in
• exciter_signal_out
• governor_signal_in
• governor_signal_out

Similarly, for governors and exciters we need to specify signal_in and signal_out. The variables communicated through the signal node also need to be specified. For example, governor signal in could be machine speed or machine current injection. This needs to be specified in the input file.

CC @superwhiskers