Implement Multiplication and Division of Quantities

[volfco]

As the title says. I implemented Multiplication and Division

[ThomasK33]

Sorry, I somehow missed this PR. Very nice, thank you.

[ThomasK33]

I'll probably also add some more syntactic sugar so that one would be able to do:

        let exp_result = ParsedQuantity {
            value: Decimal::from_f32(2.0).unwrap(),
            scale: Scale::Kilo,
            format: Format::BinarySI,
        };

        let q1 = ParsedQuantity {
            value: Decimal::from_f32(4.0).unwrap(),
            scale: Scale::Kilo,
            format: Format::BinarySI,
        };

        let q2: Decimal = 2.into();

        let result = q1 / q2;

        assert_eq!(result, exp_result);