Change trigonometric functions to return ratio instead of bare values

[adamreichold]

Fixes #185

[adamreichold]

@iliekturtles While doing this, I was a bit surprised by the test

#[test]
fn from() {
    let r1: Angle<V> = Angle::<V>::from(V::one());
    let r2: Angle<V> = V::one().into();
    let _: V = V::from(r1);
    let _: V = r2.into();
}

for Angle. Shouldn't these kind of transformations be limited to Ratio as the above basically assumes radian (resp. the base unit)?

[iliekturtles]

Thanks for the PR! Initial review from my phone looks good. I'll plan on merging once I can do a quick review from a computer.

For the From/Into impls I can't remember the history off the top of my head. Once I'm back at a computer I'll review the history and consider removing.

[iliekturtles]

Merged! Thanks for your patience. I'll look at your other PR next.

For the From/Into impls they were included with the original PR to add Angle. There was no discussion about them and perhaps they were copied from the Ratio implementation. I've add #188 to discuss removing them.