-------------------
* Add Particle Data Group significant figure auto selection feature,
documentation, and tests.
* **[BREAKING]** Use the larger of value or uncertainty to resolve the
exponent when formatting value/uncertainty pairs. The previous
behavior was to always use the value to resolve the exponent, but this
behavior was not convenient for the important use case of zero value
with non-zero uncertainty.
* Expose ``AutoPrec`` and ``AutoExp`` sentinel classes so that users can
explicitly indicate automatic precision and exponent selection.
----