minor cosmetic tweak.
Anisotropix GGX work.
Moved ParamArray::merge to Dictionary base class.
Fixed wrong comment.
Merge pull request #464 from est77/est

Moved ParamArray::merge to Dictionary base class.
CI build finished and tests passed.
Cherry pick FixedEntityModel work.
Small fixes and improvements
Merge branch 'master' of github.com:appleseedhq/appleseed
minor cosmetic tweak.
Merge pull request #465 from est77/est

Added FixedModelEntity, MaterialCollectionItem and other changes needed for GSoC
fixed compilation on Windows.
Merge branch 'master' of github.com:appleseedhq/appleseed
fixed compilation on Windows plus a few fixes and cleanups.
Merge branch 'master' into anisotropic_ggx
Added anisotropic GGX, small fixes and formatting.
Small improvements to MDF2 tests.
Small fixes.
Improved tests, fixed nans and other small changes.
Formatting fixes.
Merge pull request #466 from est77/anisotropic_ggx

Microfacet2 improvements and anisotropic GGX