Partially cleanup inc_curr_1vb_split

[JamesMcClung]

Major/notable changes:

1. Use new Vec methods in inc_curr_1vb_split
2. Use Vec3 in many places where array pointers were previously used
3. Cite the source of each current deposition technique in comments

[germasch]

👍

As you were replacing gt::sarray with Vec3, this kinda reminded me that actually it'd be great to have Vec3 to just be an alias for the former. Eventually, as usual, with some specific exceptions (like cross really only makes sense for length-3 arrays).

I could have realized that earlier, when you added various capabilities to Vec3, though I'd admit that even me myself would be somewhat scared to touch gtensor right now, as it's been sitting pretty much unmaintained for quite a while...

[JamesMcClung]

The alias thing occurred to me too, but I certainly wasn't going to touch gtensor. And yeah, cross is a bit of a hack right now.