getDispatchMethodS3: Gets the S3 method that a generic function would call
Description
Gets the S3 method that a generic function would call according to an S3 class() vector.
Usage
# S3 method for default
getDispatchMethodS3(methodName, classNames, ...)
Arguments
methodName
A character string specifying the name of a
    generic function.
Value
Returns a function, or throws an exception if not found.