Add current injection models for DERs as an model option

[sibyjackgrove]

Is your feature request related to a problem? Please describe.
Add current injection models for DER that would provide dynamics similar to the DER_A model but which would take lesser solution time.

Describe the solution you'd like
The model would only replace the system of ODEs. The ride-through and other logics would remain the same as the dynamic phasor model.