Add plot to check qualitatively the centering method

[carnisj]

In the presence of hotpixels, some centering methods are not performing well (typically, those based on the center of mass). A plot of the peak position determined by each of the methods 'max', 'com' and 'max_com' is now saved automatically to help checking the validity of the parameter setting for centering_method.

Fix #288

• New feature (non-breaking change which adds functionality)

Checklist:

• I have made corresponding changes to the documentation
• I have run doit and all tasks have passed