Format arbitrary unit nicely

[jl-wynen]

Currently, the following

auto u = units::unit_from_string("arbitraryunit");
std::cout << to_string(u) << '\n';

prints CXCUN[1]. It would be nice if it printed arbitraryunit or something along those lines.

[phlptp]

There are a lot of units that can be converted from a string but aren't displayed in an equivalent fashion.

addUserDefinedUnit("arbitraryunit", units::unit_from_string("arbitraryunit"));

should work, or whatever string you do, though that particular piece of code would only work once.

I am also thinking to add an addUserDefinedOutputUnit for cases like this as well in the near future.

[jl-wynen]

Ok. Maybe I'm biased because arbitrary unit is relatively important for us.
We have our own mechanism for handling user defined units as mentioned in #271. I opened this issue because we would prefer to avoid adding special handling to always enforce a certain formatting of CXCUN[1]. But I see your point

[phlptp]

Fair enough, easy enough to add output units like this in the tables for units that don't interfere with existing ones. Feel free to make a PR adding it, or just let me know what string you want and I will add it next I am tinkering with that part of the code. It would go in defined_unit_names_si then you just have to adjust the array size.

[SimonHeybrock]

Looking at https://en.wikipedia.org/wiki/Arbitrary_unit#Abbreviations_and_nomenclature, I would suggest to use "arb. unit", as it is the least ambiguous?