[Improve Concept Exercise] : Currency Exchange (numbers)  #2490

The currencty-exchange concept exercise for numbers needs some love.

In particular:

• Spell checking
• Language checking
• Review for readability and topic coverage
• Review of code examples for syntax and PEP-8
• Review of exemplar.py for syntax and PEP-8
• Review of test files and stub files fo syntax and PEP-8
• Addition of docstrings in stub files similar to the ones here

Optionally, if a way can be found to add complex numbers to the exercise, that would be excellent.

Exercise folder can be found here: currency-exchange
Concept folder can be found here: numbers