Allow custom units via Unitwise#register

[joshwlewis]

There have been numerous requests for adding one-off, custom, or unsual units to Unitwise. I've not supported these requests in the past because:

1. This gem is an implementation of UCUM, which curates a canonical source of units, and any units added to this gem are likely to conflict (or may in the future).
2. I don't want to maintain a list of units and their conversion values. This is exactly what UCUM exists to do.

However, there is value in supporting user-defined units. Some domains have legitimate units that don't make sense in UCUM.

This PR supports user-defined units via Unitwise.register(). It takes a hash of atom properties. For example, if your app had to measure bourbon in fingers ("three fingers of whiskey, please"):

Unitwise.register(
  names: ["finger", "fingers"],
  symbol: "🥃",
  primary_code: "fng",
  secondary_code: "fng",
  scale: {
    value: 1.0,
    unit_code: '[foz_us]'
  }
  property: 'fluid volume'
)

And now you can convert to and from it:

Unitwise(1, "gallon").to_fingers

[dpritchett]

The code reads smoothly and the docs are even better. My only question is how user-friendly the failure cases are if say someone tries to specify subtly incorrect params to :register. Will they get a nice hand-holding failure response that makes it obvious where to go next? I can't tell just by reading this PR.

 Unitwise.register(
   names: ["finger", "fingers"],
   symbol: "🥃",
   scale: {
     value: 1.0,
     unit_code: '[foz_us]'
   },
   property: 'fluid volume'
 )


Unitwise::AddRegisterError: "Lol u forgot poland 🇵🇱.  And :primary_code. And :secondary_code."
... stacktrace goes here ...

>

Maybe something like that?

[dpritchett]

I'm also wondering if you might want to (not now, YAGNI) add a set of self.list_custom_units introspective functionality one day if people start adding in custom packs of made up stuff for video game minmaxing or some such wildness.

[joshwlewis]

Ok, thanks for the feedback. I went ahead and added Unitwise::Atom#validate! which Unitwise.register uses. Any errant input should now be caught quickly.

As for listing custom units, we have Unitwise.search, so that's probably good enough for now.

[pinkynrg]

I was wondering if it might be worth implementing some check if unit already exists in the atom list. Maybe by searching for primary/secondary code? In case it exists we should either throw an exception "atom already registered" or just a plain message "atom updated!" in case we want it overridable.