tweaked formatting.
cleaned up Fresnel reflectance calculations.

also:
  - adjusted functions signatures to avoid unnecessary copies when dealing with voluminous spectra.
  - added a variant of foundation::fresnel_dielectric_schlick() that takes indices of refraction instead of normal reflectance.
fixed major bug in specular BTDF, and simplified code.

we were missing an eta^2 factor in the expression of refracted radiance in the non-adjoint case.