Correction erreurs and add of SSIM metrics

[Marien-RENAUD]

Do following modifications :

• Correct the folder to save images. (for the option --extract-images)
• Save the images for each kernel of deblurring in a separate folder.
• Add SSIM metric (It has been added for each inverse problem).
• Remove the mandatory of option --noise_level_img, especially useful for the inpainting experiment and put the default level of noise at 5.
• Save the PSNR list of z (after denoising) and not of x (after data fidelity step).