Merge pull request assimp#941 from assimp/feature/issue_940

Feature/issue 940

Version: prepare 3.3 version.

CMake: set version number to 3.2

Update doxygen documentation. Drop custom stylesheet.

Merge branch 'master' of https://github.com/assimp/assimp

Release candidate 2 : 3.1

Signed-off-by: Kim Kulling <[email protected]>

The first release candidate for 3.1.