Power dropped in format of user defined unit

[jl-wynen]

Given the unit Å^2/barn^2, to_string correctly formats this as "1/am^2". However, when defining custom units for "Å" and "barn", Å^2/barn^2 is rendered as "Å/barn^2". That is, the square on Å is lost.

Also, am is now incorrectly rendered as "1/Å/barn".

The custom units are defined as

units::addUserDefinedUnit("Å", units::unit_from_string("Å", units::strict_si));
units::addUserDefinedUnit("barn", units::unit_from_string("barn", units::strict_si));