bugfix:
- add global namespace statement to sqrt.

refactoring:
- include cmath instead of math.h in c++ API.

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>
bugfix:
- fix redundant checks against nullptr ( see https://github.com/assimp/assimp/issues/241 ).

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>