Add support for multi-dimensional attributes

[matz-e]

Essentially re-opening on #5

Using one-dimensional attributes for positions is fine, but for rotations we would prefer to use quaternions as has been the case in the past. From the official SONATA documentation:

users are strongly encouraged to only use quaternions due to its non ambiguity

Thus we should support multi-dimensional attributes. Alternatively, we could attempt to change the SONATA specification to allow orientation_{w,x,y,z} (I think this would be preferable?)

[matz-e]

@mgeplf any preferences here? To be consistent, we already have 1-D positions for everything else… I can open a PR with the official SONATA folks.

[mgeplf]

I have a strong preference for quaternions (which is an aside).

Breaking out the attribute names reduces confusion, which I'm for, so I think that's a good proposal.

Idle thought: I wonder whether we can always get away with breaking multiple-dimension attributes apart - so far it has been the case, and so far it has always made sense.

[matz-e]

See AllenInstitute/sonata#104

I will operate on the assumption that this will be merged (once it's merged, we can close this again).