prtRv Base class for all prt random variables
 
    This is an abstract class from which all prt random variables
    inherit. It can not be instantiated. prtRv contains the following 
    properties:
 
    name           - Name of the random variable.
    userData       - Structure for holding additional related to the
                     random variable.
    nDimensions    - Number of dimensions of the vector space
                     represented by the random variable.
 
    The prtRv class has the following methods
 
    plotPdf - Plot the pdf of the random variable
    plotCdf - Plot the cdf of the random variable
 
    The prtRv class has the following methods, most of which are
    overloaded. If a method is not overloaded, it is because it is not
    possible to implement the functionality.
 
    pdf - Output the pdf of the random variable evaluated at the points
          specified
 
    logPdf - Output the log-pdf of the random variable evaluated at the
             points specified (for many distributions, this can be
             calculated more easily than simply log(pdf(R,X))
 
    cdf - Output the cdf of the random variable evaluated at the
          points specified
 
    draw - Draw samples from the random variable
 
    mle - Perform maximum likelihood estimation of the objects parameters 
          using the specified data