uint64 to float64 refactoring

[divyashie]

- Summary
Fixed issue #153.
Motivation: This fix will help increase precision in the calculations on the gocommerce platform.

- Test plan
Make test
In observation of the test cases expected values and the exactness of the new floating point dollar amounts, I believe it is appropriate to change Assert.Equal to Assert.InDelta. The latter assert, which takes in a Tolerance value, allows for comparing the expected value and actual value under a certain precision.

- Description for the changelog
uint64 to float64 refactoring

[mraerino]

Thank you for looking into this.

I have to strongly disagree with the decision to turn every amount into a float.
Floats have several issues and because of rounding anomalies are not a good fit for calculating monetary amounts.

Some articles around this:
https://husobee.github.io/money/float/2016/09/23/never-use-floats-for-currency.html
https://spin.atomicobject.com/2014/08/14/currency-rounding-errors/

You also used a float on values that really don't make any sense with decimals (e.g. quantity or DiscountItem.Fixed).

We definitely have to find a way to represent decimals of any length in some cases (percentage discounts & taxes), but I see no purpose in turning everything into a float, especially not currency amounts where it might be harmful.

[bcomnes]

@divyashie Thank you very much for the PR, interest and time! I'm going to close this approach for now for the reasons @mraerino mentioned, but if you are still interested on the problem, lets discuss ideas in #153