def coerce(other)
      return [other.to_unit, self] if other.respond_to? :to_unit
      case other
      when Unit
        [other, self]
      else
        [RubyUnits::Unit.new(other), self]
      end
    end
def coerce(other)
      return [other.to_unit, self] if other.respond_to? :to_unit
      case other
      when Unit
        [other, self]
      else
        [RubyUnits::Unit.new(other), self]
      end
    end
automatically coerce objects to units when possible
 if an object defines a 'to_unit' method, it will be coerced using that method
 @param [Object, #to_unit]
 @return [Array]