ENH: Denoise all anatomical images, but outside of loop

[cookpa]

Before:

• Only the first image gets denoised
• Denoising happens at every iteration

Now:

• Denoise all images
• Do denoising once outside of the loop

Also added a check for prior images - if priors are incorrectly specified, exit with error instead of carrying on with KMeans.

[ntustison]

Thanks @cookpa