Complex user defined unit rendering not working

[jl-wynen]

We use the unit meV^2 and it gets rendered as 2.5669699665355694e-44*J^2 using to_string(unit). We tried using a user defined unit:

addUserDefinedUnit("meV^2", unit);

but that does not affect how the unit is rendered.

Similarly, user defined unit do not seem to work for other squared units. We tried 1/meV^2 and barn^2.

I'm not sure if this is a bug or if there is just a special case missing in the formatter.