Merge pull request #520 from est77/master

Made refraction tests more robust in microfacet BTDF, fixed bug.
Cosmetic fixes for previous commits

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Merge pull request #521 from marius-avram/cosmetic_fixes

Cosmetic fixes for previous commits
Added some sample seexpr expressions.