Ellipsoid VTK output

See merge request walberla/walberla!85

External apllications which did not set the C++ standard to 14 in their CMakeLists.txt failed to build. This change allows them to build without further user actions.

Require C++14 and CMake 3.1

See merge request walberla/walberla!79

- Avoid that defining points are aligned on a Plane
- Improve numerical stability of cross product