Hello world in ex1.cpp
ex2.cpp: a more interactive program

Ask two numbers to user and compute product
ex3.cpp Adding more functionality

Ask the user two numbers and an operation. Check for error on cin.
ex3.cpp: more tests for error

Test for division by zero. Do not output result if error.
ex4.cpp: while loop introduced and better error treatment
Merging everything into ex1.cpp
Implementing functions for operations.
Implementation of intercept.

Plus a bit of reshuffling, create functions and support for any number
of input number and longer operation names.