Support for OffsetArrays

[piever]

Currently this gives:

julia> using OffsetArrays

julia> VectorOfArray([OffsetArray(rand(2), 0:1)])
ERROR: size not supported for arrays with axes (0:1,); see http://docs.julialang.org/en/latest/devdocs/offset-arrays/

but in principle I guess it could work (maybe Compat.axes should be used instead).

[ChrisRackauckas]

replace many uses of size with indices

That's probably all we need to do.