The "mixture" project is useful, if the following is true:
  * you need to simulate a process in a complex heterogeneous 3D domain 
    consisting of inclusions of different size, orientation and distribution;
  * you plan to mesh this domain using Gmsh.

The project is quite simple, so its main goal is to create a .geo file, that 
can be used further as input for Gmsh. There are limitations for a specimen 
shape and for inclusions shapes and distribution:
  * specimen is a brick only, with edges parallel to Cartesian axes;
  * inclusions are
    * orthobricks (bricks with edges orthogonal to each other);
    * cylinders (with ellipse or circle as base);
    * ellipsoids;
    * spheres (a particular case of ellipsoids);
    * standard tetrahedra (all edges are equal).
  * inclusions cannot intersect each other and the faces of the specimen.

So, this project is useful if you model such media as concrete, porous media, 
composite material with solid inclusions that have special shape and cannot 
intersect each other.