name matching between params_timevar and opt_pars$time_params

[bbolker]

if opt_pars$time_params is named, substitution into params_timevar$Relative_value should be done by name, not position, following the convention that the names will be [Symbol].[Date] (which should be unique).

Also, we should have a check that length(opt_pars$time_params) == sum(is.na(params_timevar$Relative_value)).