Introducing DecimalMath

This adds functionality for doing basic arithemetics, rounding, etc
on DecimalValue objects. This is useful for automatically determining
precision/uncertainty based on the number of digits given, and ultimately,
for unit conversion.

Change-Id: Idc3006d7e0f5fd635233a3de57b8194f76f4b434
Introducing QuantityValue::transform

This introduces a general purpose method for transforming quantities,
keeping the amount, bounds and number of digits consistent.

Change-Id: I3c49e98a9183c0af000bfbf1f5753f4396d1dad3
Merge "Introducing DecimalMath"
Merge "Introducing QuantityValue::transform"