Bugfix/absolute value

[jccockre]

A couple of points here:

• Since Abs() returns an UnknownUnit rather than void, I assume the return value is supposed to contain the absolute value. But I found the underlying value was also mutated -- calling Abs() on a negative quantity changed it to a positive quantity, so I lost my original value. This is noted in test case 01 and is resolved by my pull request.

• Not resolved by this pull request, I think it would be desirable if the unit prefix was fully preserved through this operation. So the absolute value of -32 MHz could be written by the ToString() method as 32 MHz rather than -3.2e7 Hz. I understand this is a broader change and may not be considered urgent or important right now.

[MadsKirkFoged]

You are right! My intention with not to mutate the original object when calling .abs()

I will have a look at fully preserving it through the operation - Its the right thing to do