Calculate Reference Yield

## Arguments

- x
Integer, the simulation number

- StockPars
List of Stock Parameters

- FleetPars
List of Fleet Parameters

- pyears
The number of years to project forward. Equal to 'nyears' for optimizing for q.

- Ncurr
Array with current numbers-at-age (dim=c(nsim, maxage+1, nareas))

- nyears
Number of historical years

- proyears
Number of projection years