Support unit lists

[bsidhom]

It would be great if Numbat supported unit lists out of the box à la Gnu Units. This allows units to be displayed in an effectively arbitrary "base" consisting of other units.

I understand that this may be tricky or conceptually incompatible with unit typing, but maybe it could make sense to have some short-hand to define unit list functions that convert to String. I'm thinking of something similar to the human() time function, which currently does this manually in a pretty janky way. (Perhaps the real issue is that custom functions don't have the greatest ergonomics; I'll start a discussion expanding on that.)

Could be nice to have proper support for unit lists as typed entities, though I haven't put much thought into whether that makes sense.

[sharkdp]

Qalculate calls this feature "mixed units". You can convert heights to "feet + inch" and times to "hours + minutes + seconds", etc.

I agree it would be nice to support this out-of-the-box, without having to write functions like human.

this may be tricky or conceptually incompatible with unit typing

Good thing Numbat is not based on "unit types", but on "dimension types" 😄 [1] [2]. But seriously: I don't think our type system is going to be the problem. If anything, it helps. It makes sense to convert a height to [feet, inch] where all units are of type Length. Or a time to [hours, minutes, seconds] where all units are of type Time. It does not make sense to convert a length to, say, [feet, second].

Concerning syntax, I'm thinking we could do one of the following

length -> [feet, inch]
length -> feet + inch

The latter might be more intuitive, but the former makes more sense from a language-design perspective, I believe.

[1] https://numbat.dev/articles/intro.html
[2] https://numbat.dev/doc/type-system.html