Categories for rulinalg

[Andlon]

I just noticed through this reddit link that crates.io has support for categories, although this does not seem to be accessible from the front page yet. I suspect this is just a matter of time.

In any case, I looked through the available categories, and I think Rulinalg should be present in the "Science" category. I think this is just a matter of adding

categories = [ "science" ]

to the [package] section of Cargo.toml.

I would make a PR myself, but I'm not on my dev computer, and I really should be doing other things right now anyway, heh. Just wanted to note it down before I forgot. Maybe I'll get it done later.