Macro units 

macro_rules! units {
    (Percent) => { ... };
    ($base_unit:ident) => { ... };
    ($prefix:ident, $base_unit:ident) => { ... };
}

Assembles fully-specified measurement units for NumericStateRecorder, combining a base unit with an optional prefix.

Examples: - units!(Volt) - units!(Percent) - units!(Kilo, Hertz) - units!(Milli, Amp)