Fix calc_normal for edges

See merge request !325

Add missing include.

See merge request !324

OBJWriter performance: Add map from color to material index, avoids quadratic performance

See merge request !323

use calc_face_normal if no normal property is available

See merge request !321

ModEdgeLength Fix incompatibility with Eigen vector type

Closes #81

See merge request !320

closes #81

Fix some "unused parameter" warnings.

See merge request !319

- added param 'texture_file' to the Options class, it specifies the path to the texture file

See merge request !318

- added param 'material_file_extension' to the Options class, it specifies the material file suffix, default is ".mat" as it was before, we needed it becuase Blender expects ".mtl"
- removed default and copy constructor and destructor as well as assignment operator, because these are not necessary here, see https://en.cppreference.com/w/cpp/language/rule_of_three

(Thanks to Philipp Auersperg-Castell for the patch )

Removed duplicate cmake code

See merge request !317

Prefer GLVND OpenGL

Cmake jan

See merge request !316