Series Hybrids: add `HybridArchitecture` enum for HybridElectricVehicle

FASTSim models all HEVs as parallel hybrids, where the power being supplied to the transmission can come from a combination of EM & FC

We should introduce a new field on HybridElectricVehicle: an enum that allows us to explicitly model different hybrid architectures:

pub struct HybridElectricVehicle {
    // ...
    #[serde(default)]
    pub architecture: HybridArchitecture,
    // ...
}

pub enum HybridArchitecture {
    // default option, for compatibility with existing models
    Parallel,
    Series { generator: ElectricMachine },
    // future work:
    // PowerSplit { motor_generator: ElectricMachine },
}

impl Default for HybridArchitecture {
    fn default() -> Self {
        Self::Parallel
    }
}

This will need to be matched on in both directions to add new logic:

• set_curr_pwr_prop_out_max (component limit calculations, calculated power sources -> wheels)
• solve (power flow, calculated wheels <- power sources)

Necessary for #270, as foundational conceptual work rather than necessary for FuelCellElectricVehicle