Allow exact calculations for fractional transfers

[NCGThompson]

I don't know of any specific jurisdictions that require this, but it makes sense to allow the user to opt-out of decimal rounding for vote transfers by defining decimalPlacesForVoteArithmetic as a specific value (such as -1).

[NCGThompson]

I edited out some bad math, but my point remains.

[tarheel]

Thanks for the suggestion! We're not aware of any jurisdictions that would use this either, but it sounds like something that wouldn't be too hard to add if/when we hear of a real-world use case.