Loosen bounds in pick_appropriate_units

[alberdingk-thijm]

Change strict inequality to inequality for the inner loop of
pick_appropriate_units, so that if a value is set to 1.0, then
the unit returned follows a user's expectation.
Closes #24.

[eldruin]

Thank you for this PR!
Could you add some unit tests for the behaviour?
Could you add an entry to the changelog?

[alberdingk-thijm]

@eldruin added the tests and changelog entry! Let me know if they conform to the project's style; happy to make edits.

[eldruin]

Thank you for the changes!
I added some nitpick comments. The most important for me would be more descriptive test names.
Other than that looks good to me.
In case you want to play around with macros, the code could also be simplified with something like this:

macro_rules! test_from_kg {
    ($name:ident, $value:expr, $unit:expr) => {
        #[test]
        fn $name {
            let mass = Mass::from_kilograms($value);
            let (unit, v) = mass.get_appropriate_units();
            assert_eq!(unit, $unit);
            assert_almost_eq($value, v);
        }
    };
}

[alberdingk-thijm]

Whoops, sorry for letting this languish. Thanks for the review, @eldruin, I've made the changes you suggested.

[eldruin]

Looks great to me. Thank you!