Improved color aberration auto-correction

I believe this branch is ready for integration.

Main changes :
* No loss of color saturation
* Smooth color shifts (no block discontinuities)
* Significantly more accurate measure to estimate color shifts
* The fitting has been fixed and improved
* Restriction to radial correction is an option
* Restored OpenMP support
* Improved border reconstruction
* Clean code

It's activated on export, and in the GUI in zoomed mode by default.
I believe the current code is better than the legacy code in every aspect. So enjoy!

also set it to NULL