Add Frequency Reference Options for Simulation

[rodrigomha]

Including Center-of-Inertia for system frequency, or using Frequency-Divider approaches to obtain frequency for each bus that can be passed directly to inverters (skipping PLL implementation).

[m-bossart]

@jd-lara Would this change also allow for using a device as the frequency ReferenceBus that doesn't have frequency as a dynamic state (Droop GFM)?

[jd-lara]

We can account for that as an option. This is an interesting question about how to implement it. We can add another option.

[m-bossart]

Yeah it seems like the current implementation is limited to devices that have frequency as a state that is therefore already stored in the result.