[calib2] Persist the prefered "method" in the configuration

[kif]

Persist in the configuration file ... all is already foreseen by @vallsv To be persisted in the same way:

• [ ] Integration method
• [ ] OpenCL device selection
• [ ] Polarization factor
• [ ] Number of azimuthal points
• [ ] Number of radial points
• [ ] Integration unit

and maybe few others ....