Add LatLongCalculator

Needed in https://github.com/wmde/WikibaseQueryEngine/pull/54.

I give up. I tried to come up with a public `equals` that just works but it's hard. Values about 360° need a few bits of precision before the comma. Values about 0° can use these bits for additional decimal places after the comma. Always rounding these decimal places away (basically applying `round( ..., 13 )`) doesn't make much sense. Why lose precision if it's there?

I dropped half of the code I wrote.

To make this clear: We are _never loosing_ precision. We are _gaining_ precision. But the additional decimal places are pointless.
...and I wrote DecimalMath for QuantityValue :)