Briefly prints a couple of lines from str to avoid swamping the console with
the contents of very large objects.
Usage
# S4 method for class 'Data'show(object)# S4 method for class 'OM'show(object)# S4 method for class 'Hist'show(object)# S4 method for class 'MSE'show(object)# S4 method for class 'MMSE'show(object)
