Fixed decimalExpansion

[tingwoo]

I think I've fixed the issue with decimalExpansion rounding. (issue #72)
Now, when inputting 7450921770987 / 7450936270987

let num = BDouble("7450921770987", over: "7450936270987")!
print(num.decimalDescription)
for i in 0...8 {
    print(num.decimalExpansion(precisionAfterDecimalPoint: i))
}

it'll give

1.0000
1.
1.0
1.00
1.000
1.0000
1.00000
0.999998
0.9999981
0.99999805

which are the correct answers.

[mkrd]

Hey @tingwoo thanks a lot! I would be super thankful if you could add these to the test cases as well, so we know for sure that future changes will not break this!

[tingwoo]

Done! I didn't notice that BDouble("2")!.decimalExpansion(precisionAfterDecimalPoint: 0) should give 2.0 instead of 2, so I also implemented that in the last commit.
By the way, I can't get the tests to run on my machine (it says "No such module 'MGTools'"), do you perhaps have any ideas on why?

[mkrd]

Thanks for adding the tests, great! I'm not sure why you get the error, but I will be looking into it