Macro mag::declare_unit

macro_rules! declare_unit {
    ($(#[$doc:meta])*
        $unit:ident,
        $label:expr,
        $measure:ident,
        $factor:expr,
    ) => { ... };
    ($(#[$doc:meta])*
        $unit:ident,
        $label:expr,
        $measure:ident,
        $factor:expr,
        $zero:expr,
    ) => { ... };
}

Define a custom unit of measure.

• unit Unit struct name
• label Standard unit label
• measure A base or derived measure
• factor Factor to convert
• zero (Absolute) zero point