Use os.system instead of subprocess.check_call for FSLeyes and ITKsnap commands

[valosekj]

FSLeyes often prints a lot of warnings into the CLI (for example, when switching between overlays, changing colormap, etc.). The usage of subprocess.check_call (introduced in 8aa0e3a to replace os.system) terminates the manual_correction.py script in such cases.

This PR thus bring back os.system for FSLeyes and ITKsnap commands.

[NathanMolinier]

I understand the issue, however I still believe that using os.system is not good (because we are not able to catch ctrl+c commands). To fix this issue I thereby think we should use subprocess.run as done in this example:

 out=subprocess.run(['sct_label_vertebrae',
                                        '-i', img_path,
                                        '-s', seg_path,
                                        '-c', 't2')
                if out.returncode == 0:
                    # no problem detected
                elif out.returncode == codeWarning:
                    # no problem detected
                else:
                   # different problem detected

So we need first to identify the code of this warning.