'XPMType' - 'auto' (default) / 'TwoThirds'
    'auto' - tries to select between the following two options:
    'TimesTwoThirds' - 2/3 of gamma is use, this is OK assuming
        XPM between two perpendicular polarizations with the
        same frequency in isotropic medium.
    'TimesTwo' - 2 gamma is used, this is OK for XPM in an
        isotropic medium between two parallel polarizations of
        different frequencies
    'Chi3Matrix' - chi3 matrix is used for calculations (not
        implemented)