Better explain planar/non-planar axes

[hinerm]

I think this concept is very confusing, as the phrase "planar axis" conveys an idea of "contributing to plane count" to many people. At the least it a there should be a good tutorial explaining this distinction and why it is this way (abstraction that was necessary to handle interleaved/non-interleaved/non-planar Channel scenarios)

[ctrueden]

This will fundamentally change with the work of scifio/scifio#283.