Measurementの定義
/// A `Measurement` is a model type that holds a `Double` value associated with a `Unit`.
/// Measurements support a large set of operators, including `+`, `-`, `*`, `/`, and a full set of
comparison operators.
public struct Measurement where UnitType : Unit {
/// The unit component of the `Measurement`.
public let unit: UnitType
/// The value component of the `Measurement`.
public var value: Double
/// Create a `Measurement` given a specified value and unit.
public init(value: Double, unit: UnitType)
}
(一部略)