Generic function that computes abundance within the covered region.  It
calls method (class) specific functions for the computation.
     
    
    Usage
    NCovered(par, model = NULL, group = TRUE)
 
    
    Arguments
- par
- parameter values (used when computing derivatives wrt parameter
uncertainty); if NULL parameter values in - modelare used
 
- model
- ddf model object 
- group
- if TRUE computes group abundance and if FALSE individual
abundance