Added more in depth description of gamma correction

[jeromelecoq]

Hi all,
I added more details on a dedicated page here : https://allenneuraldynamics.github.io/openscope-community-predictive-processing/stimuli/gamma-calibration/

It contains :

• Code for calibration
• Code fot test
• An example bmp calibration
• And before and after picture to visualize the process.