Make Zygote more Julian

[awf]

1. make cam in project a

struct Camera{T}
   rot :: SVector{3,T}
   centre :: SVector{2,T}
   radial :: SVector{2,T}
   focal :: T
   X0 : SVector{2,T}

[awf]

2. replace pack() unpack() with struct as above

[awf]

3. Replace parents integer indices with a tree data structure. This will allow for less mutative updating in relatives_to_absolutes and cleaner, hopefully faster, code.