Characterize posterior predictive data

## Arguments

- PPD
An object of class Data stored in the Misc slot of an MSE object following a call of

`runMSE(PPD = TRUE)`

.- styr
Positive integer, the starting year for calculation of quantities

- res
Positive integer, the temporal resolution (chunks - normally years) over which to calculate quantities

- tsd
Character vector of names of types of data: Cat = catch, Ind = relative abundance index, ML = mean length in catches

- stat
Character vector of types of quantity to be calculated: slp = slope(log(x)), AAV = average annual variability, mu = mean(log(x))