Make AtomView rely on struct-of-array format

[Liozou]

Hi! Here is a proposal to simplify a little bit the definition of struct-of-array systems, like FastSystem.

Currently, the two following definitions are required for each property like position:

position(s::FastSystem) = s.position
position(s::FastSystem, i) = s.position[i]

Since they are quite redundant, I suggest not requiring the latter. Instead, a position(system, i) call would be intercepted by the already existing fallback

AtomsBase.jl/src/interface.jl

Line 100 in 0927561

position(sys::AbstractSystem, index) = position(sys[index])

which can be made to work simply by tweaking the definitions of position(v::AtomView). The emitted code should be identical in the end.

Also add some documentation for AtomView (since it is already exported and mentioned in the docs).

[mfherbst]

Very nice. I think we should've done this in the first place.

[Liozou]

Great, thanks for the review!