Draft: Plot and save COR estimation cost func

This MR adresses issue #515. If verbose==True in the estimator object:

• A cost.npy file is created in a temp dir (not sure it's the best thing to do).
• A plot is displayed. NB: implemented for sliding, growing, fourier. centered and octave are 2D algos (based on 2D correlations).

Close #320, #515