indexing into a 3D array has unexpected overhead

[samtkaplan]

accessing a 3D array using 3D indexing is ~3X slower than indexing into a 3D array using 1D (linear) indexing. Please refer to the mailing list discussion here:

https://groups.google.com/d/msg/julia-users/aYS_AvKqPCI/DyTiq4lKIAoJ

and the gist, here:

https://gist.github.com/samtkaplan/5991236

Please note that, in the gist, the 1D indices are explicitly computed from the 3D indices. That is, the overhead of converting 3D indices to 1D indices is included in the 1D indexing example.