Power dropped in format of user defined unit

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

Reproduce the Å^2/barn^2 and am cases with the C++ example in the issue, then trace custom-unit registration and the to_string formatting path. Add a regression test for both expressions; done means the exponent on Å is preserved and am is rendered correctly after defining the custom units.

Written by the indexing model from the issue text.