Optional average PSF over whole image

[tibuch]

From #61:

Also, would there be a way to get a tick box asking if we want to have an average PSF with SD to see the homogeneity of the FOV.

[rdemets]

This is because I noticed that software such as Huygens PSF distiller or Fiji plugin MetroloJ tend to give an average PSF fit as well, which I find quite useful if we wish to summarize microscope performances to our users.