Modified Bobbert_Vlieger_BRDF_Model, Subsurface_Bobbert_Vlieger_BRDF_Model, Axisymmetric_Particle_BRDF_Model, and Subsurface_Axisymmetric_Particle_BRDF_Model by removing the functions PartialExtinctionS(double theta) and PartialExtinctionP(double theta) (which provided a partial extinction coefficient) and replaced them with a single function Specular(double theta) that returns the Mueller matrix specular reflectance or regular transmittance, calculated using the optical theorem.
Repaired errors in MuellerMatrix::valid() and MuellerMatrix::physically_valid().
Modified Polydisperse_Sphere_BRDF_Model to add a parameter fractional_coverage that, if non-zero, renormalizes the distribution to a given fractional coverage of spheres on the surface.
Modified CC1246E_SurfaceParticleSizeDistribution so that, the distribution is renormalized to the same fractional coverage of spheres on the surface (with sizes from 1 µm to 1000 µm) as when slope = 0.926 with the given value of cleanliness.