Implement composite units

[ronaldtse]

A benefit of UnitsML is the dynamic composition of new units based on root units.

This functionality is implemented in https://github.com/plurimath/asciimath2unitsml already as described by unitsml/unitsdb#27 (comment) .

e.g. To support AsciiUnits syntax:

unitsml(
  composite: watt * meter^-1 * steradian^-1, 
  quantity: radiance
)

We need to be able to do such composition in Ruby like:

Unitsml::Unit.new(
  units: [
  {
    unit: Unitsml::Unit::Watt,
    power: 1
  },
  {
    unit: Unitsml::Unit::Meter,
    power: -1
  },
  {
    unit: Unitsml::Unit::Steradian,
    power: -1
  }
  ],
  quantity: Unitsml::Quantity::Radiance
}

Or in short "chained" form:

Unitsml::Unit.new.
  add_root(Unitsml::Unit::Watt, 1).
  add_root(Unitsml::Unit::Meter, -1).
  add_root(Unitsml::Unit::Steradian, -1).
  set_quantity(Unitsml::Quantity::Radiance)