Update installation guide for linux and python.

closes https://github.com/assimp/assimp/issues/1758: fix compiler war…

including <unzip.h> instead of <contrib/unzip/unzip.h>

Fix Matrix4x4t Decompose to rotation vector.

The calculation of the rotation matrix was for left-handed
coordinates with row-vectors, but assimp uses right-handed
coordinates and column-vectors.

closes https://github.com/assimp/assimp/issues/905: add missing contr…

Warning LNK4221 when not building exporter.

Update OgreXmlSerializer.cpp

- Make constats static
- add asserts to check against nullptr dereferencing

Fix thread-safety false positives in Ogre importer

They're causing false positive race condition messages from Helgrind

It's immediately passed via string pointer and in most places
it's already a raw string constant.

New license dates

Fix more thread-safety issue in 3DS and ASE loaders