Fix the way the conversion factor is obtained in serde and `Debug` impls

[Tehforsch]

The code is extremely ugly. Should be easy enough to generate code that matches all defined units, in code similar to the following

let conversion_factor =
if D == Dimension { length: 0, ..NONE } {
    match unit_symbol {
        "%" => 0.01,
        "dimensionless" => 1.0,
    }
}