slice_15_alta.dat:  the slice at 15 degrees of incidence in the
parametrization  (theta_o*cos(delta_phi), theta_o*sin(delta_phi) ) -->
BRDF * cosine_factor. Data have been rescaled

dense_slice_15_alta.dat :  dense interpolation of the slice at 15
degrees (previous file) where values below zero have been clamped to
zero.


bande_jaune_slice_15_isotropic_brdfcc_rescaled_alta.dat:  slice at 15
degrees in the original parametrization (theta_i, theta_o, delta_phi)
--> brdf * cosine_factor.   Retro-lobe has been rescaled.