Modularise calculation code

[adcwilliams]

Motivation

The current calc.py code is functional but static and hard to test. As the first step towards including new features we need to move the calculation code into a separate module, leaving the main program simply for input and output. The modules can then contain specific unit tests.

Objective

• Move calculation code into module.
• Add appropriate unit test for the existing addition function.

[adcwilliams]

@rse2019coder , get started 💰

[rse2019coder]

First step is to create a placeholder module containing correct input and output arguments,

[rse2019coder]

Now I need to move the existing addition functionality into the module.

[rse2019coder]

I need to add a suitable test for the new module, my_function.py.

[rse2019coder]

Code complete and runs successfully