Complete the implementation of the Money Model:
* Review the current implementation and make any change you consider to improve this Model.
* Add the behaviour needed to multiply a Money to a factor.
* Add the behaviour needed to convert Money to a different currency given a Date (the Date of the conversion rate).
* Finally, given a list of Items, where each Item has a Price in ARS and a Tax, it is necessary to calculate
the sum of all the Items (Price * Tax) and then convert the total to Dollars with the conversion rate of the
previous Date.