Improved saturation/mask management.

- Saturation (max) value is calculated just once, for the brightest image, and applied to the others.
  In this way, assuming that the brightest image is saturated, we can allways obtain the correct value.
- Reduce the saturation threshold to 90% of the maximum value, and use it for alignment and relative exposure.
- Calculate the mask of each image only with the saturated pixels. Then, fatten the mask with an arbitrary radius and blur.
- Experimental: If the blurred part of the mask contains saturated pixels, decrement its value further.
- Parallelize image alignment.
- Add a command line switch to modify the fatten/blur radius.
- Hide Exiv2 warnings.
Improve computeRelExposures performance
Add GUI switch for mask radius, and translations