Add field `output_power_domain`: `PowerDomain` to FuelConverter

We should introduce a field that designates the domain of the fuel converters output (electrical power or mechanical power)

pub enum PowerDomain {
    Mechanical,
    Electrical,
}

pub struct FuelConverter {
    ...
    #[serde(default = fc_output_power_domain_default)]
    pub output_power_domain: PowerDomain,
}

fn fc_output_power_domain_default() -> PowerDomain {
    PowerDomain::Mechanical
}

We can use output_power_domain during initialization to ensure the fuel converter is compatible with the vehicle architecture.

Examples:

• Conventional vehicle:
  • requires Mechanical
    • fuel converter output feeds drivetrain directly
• Parallel HEV:
  • requires Mechanical
    • fuel converter provides mechanical power alongside electric machine
• Series HEV:
  • requires Mechanical
    • fuel converter output feeds generator
• FCEV:
  • requires Electrical
    • fuel converter output feeds electrical bus directly

This prevents invalid combinations from silently producing incorrect simulations.

We can also use it for matches in the future, or for more intricate power flow modeling

Should be included in the rollout of FCEV #270