Make adding units with SI prefixes simpler

[thegedge]

I'm thinking we could add an SI helper when building a unit system, which will generate units with the SI prefixes added. Something along the lines:

si_unit :g, aliases: [:gram, :grams]
unit :g, aliases: [:gram, :grams], with_si_prefixes: true

This would generate kilograms, megagrams, micrograms, etc.

[kirinrastogi]

What range would this go to by default? 10⁹ to 10^-9 or even more?

[kirinrastogi]

I have started on this, and I don't know if the approach I am taking is good.

I was going to create a class Measured::SIUnitSystemBuilder
with def si_unit(unit_name, aliases: [], value: nil, range = 9)
The class would have two arrays, each with every 3 prefixes, and they can be as long as needed
[:k, :M, :G], and [:m, :μ, :n] would be snippets of them

Then something would iterate over these and call build_unit them, with default aliases "#{unit_name}s"
And set the value of the unit relative to the si base unit.

Am I on the right track here?
Also, I wasn't sure why with_si_prefixes: true is needed. Can't si_unit build the base unit, as well as ones with all of the prefixes?

[thegedge]

From browsing around the web it looks like there are 20 prefixes (see Wikipedia, for example), so I'd go with those.

I'll address your other questions in your (wip) PR!