EV Aggregation Model

[jinningwang]

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Implement the charging-time-constrained EV aggregation based on the state-space model.

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Additional context

References:

1. J. Wang et al., "Electric Vehicles Charging Time Constrained Deliverable Provision of Secondary Frequency Regulation," in IEEE Transactions on Smart Grid, vol. 15, no. 4, pp. 3892-3903, July 2024, doi: 10.1109/TSG.2024.3356948.
2. M. Wang et al., "State Space Model of Aggregated Electric Vehicles for Frequency Regulation," in
   IEEE Transactions on Smart Grid, vol. 11, no. 2, pp. 981-994, March 2020, doi: 10.1109/TSG.2019.2929052.