Refactor GaussianVehicleFactory input checks into separate function

[CarlinLiao]

https://github.com/spartalab/aimsim/blob/56e9beaeb158b0c045f117e3b44f2c7421879659/aimsim/endpoints/factories/gaussian.py#L48-L72

From #2 (comment)

@ribsthakkar

I'd refactor this validation into its own function, that you could separately update later and stuff given any potential changes in this

@CarlinLiao

What would that function look like? Does it take the entire blob of input arguments? If so, I don't see how that's much better than letting this init do the error checking itself.