VehicleParameters as an enum

[nreinicke]

PR #204 introduced the concept of the VehicleParameter.

@robfitzgerald points out that:

we may want to come back around to this module and relax the design of the VehicleParameters so that queries can provide a subset of the parameters, for example, when that information is not available for a vehicle. we could do this by turning VehicleParameters into an enum and deserializing a Vec from the user query. if that were the case, we could even avoid the mandatory O(n) sweep for each Vec because we could match by type only the relevant restrictions.