spectral smoothing on a single channel breaks stats calculation

[MatthewWhiting]

I was running a test on a cube, just looking at a single channel, but I still had the spectral smoothing option set. The stats are measured from the smoothed array, but this is blank, so *all* the stats came out to be zero.

Solution - if the image is 2D and spectral smoothing is selected, turn off the smooth flag (and delete the recon array, as this will have been allocated).

[MatthewWhiting]

Better - we should know at initialisation whether the image is 2D, so can incorporate that as part of the check as to whether the recon array should be allocated.

[MatthewWhiting]

Done as per second comment, and committed [817].

Closing ticket.