Given a sample of size n of positive exceedances, compute
the real shape parameter \(\xi\) based on the k largest order statistics.
     
    
    
    Arguments
- xdat
- vector of positive observations of length \(n\) 
- k
- number of largest order statistics 
 
    
    Value
    a data frame with the number of order statistics k and the shape parameter estimate shape, or a single numeric value if k is a scalar.
     
    
    References
    Pickands, III, J. (1975). Statistical inference using extreme order statistics. Annals of Statistics, 3, 119-131.