Add ChangeSimulator run method

This method iterates length simulations and returns an average of the
Customer's number of coins. The results should represent the ideal
amount of coins that a person should carry.
Raise error if Prices created with array argument with test
Indent ChangeMaker tests and adjust "end" delimeter
Add Prices load_file method

It takes a file path string and loads its contents in Prices @data. The
prices must be numbers on individual lines.