Add focus projection

[muellerflorian]

In our Nat. Comm paper (Samacoits et al.), we used a focus projection approach to remove out-of-focus slides and select pixels based on best focus in the remaining slices.

Adding this option to create 2D images might be useful for some users.

[muellerflorian]

a potentially simple solution would also be to use only one slice rather than a MIP. a simple focus measurement could be the coefficient of variation. then we just take the slice with the max value assuming that this slice in most "in focus" and contains most relevant information.