Add bean geometry

Version 1.0
Customizable bean shape geometry

* Users can customize several parameters in the geometry
    - dim: topological dimension (=2 or =3 respectively for 2D and 3D)
    - d: the bean length
    - r1: the bean left radius
    - r2: the bean right radius
* Note that you can use the Feel++ option `--gmsh.geo-variables-list` to
  modify these parameters on the fly.
* The 3D generators might crash for some shapes. See the .geo file for
  an example of bug setting. You should consider making a prior check
  using gmsh to guaranty that everything works.

Enjoy!