Rename density to mass density

[Aehmlo]

"Mass density" is the more rigorous and technically correct version (though I believe the SI brochure does essentially say they're equivalent), so I think we should at least expose this as an option.

To retain backwards compatibility, this PR contains pub type Density<U, V> = MassDensity<U, V>. I'd rather not have this from an idealistic standpoint, but from a practical perspective, it makes sense to not break backwards compatibility for this (which I would expect to be one of the more commonly-used quantities). We could also just make the type alias work the other way around (pub type MassDensity<U, V> = Density<U, V>) and thereby obtain a single-line diff (two lines with docs), but I do think that this is the more semantically correct version.

[coveralls]

Coverage remained the same at 97.229% when pulling 8a2940b on Aehmlo:mass-density into 8c5eb43 on iliekturtles:master.

[iliekturtles]

I agree that MassDensity is the better name. I can't remember why Density was chosen over MassDensity but I'll do a bit of digging when I get a moment. I think it would also be good to put a #[deprecated] attribute on the alias to the old name in order to get users to switch.

[Aehmlo]

Good idea, I’ll add that.

[iliekturtles]

Once the comments I made in the diff are resolved I'll merge!

• Update comment on kilogram_per_cubic_meter to say "... unit of mass density."

[iliekturtles]

Thanks! I manually merged and resolved the conflicts with #129.