Simplify image class data storage

[ivmartel]

The image class storage uses multi dimensional array to store possible frames. This must be simplified to only use one dimension and find frame (or other dimension) data according to input dimensions. That way frames are only used when necessary.

Allocating the full array at startup may be preferable instead of the current per slice allocation (similar spirit as #569). Simplifying code to facilitate garbage collection of slice data could also help.