treats logistic as a function of covariates; for a given covariate
combination it computes function at with those covariate values at a
range of distances
     
    
    Usage
    logisticbyx(distance, x, models, beta, point)
 
    
    Arguments
- distance
- vector of distance values 
- x
- covariate data 
- models
- model list 
- beta
- logistic parameters 
- point
- TRUEif a point transect model
 
 
    
    Value
    vector of probabilities