Fix reporting of units to be dynamic based on service and/or IMT

Y-value units for GMM distance and magnitude are reported as Ground Motion (g). These services allow selection of PGV as IMT in which case units should be cm/s. Units are available from the IMT enum, so the IMT must be provided to the service response object generator. Probably need to provide GmmService.Response.create() with Optional<Imt> that will be present if service is Id.MAGNITUDE or Id.DISTANCE or Id.HW_FW.