Suggestion: allow multiplying by a unit

[ttencate]

The dimensioned crate allows us to multiply raw float values with a unit to produce a quantity:

let length: Meter<f64> = 3.0 * M;

In uom, this would look like:

let length: Length = 3.0 * meter;

Similarly, division f64 / Unit could be useful:

let frequency: Frequency = 5.0 / second;

For consistency, also allow Unit / f64:

let one_minute = hour / 60.0;

Essentially, a value that implements Unit could be used wherever a quantity of that dimension is expected.

[jacg]

I've been meaning to propose this for a few weeks now, just never got around to it!

[iliekturtles]

Thanks for submitting the issue. This goes along with #91, but I think should be tracked separately.