v0.2.0

Release 0.2.0

• ObjectSet.mesh_step can be used to associate a mesh step with a the object set.
  Setting the mesh step is postponed right before the mesh is created otherwise the mesh step is forgotten by GMSH
  during later geometric operations.
• GeometryOCC new methods:
  operations: extrude, revolve
  primitives: circle, disc, cylinder_discrete, disc_discrete
• Complex geometry of the Greet experiment is presented as a tutorial 'tutorials/01_Greet_experiment'.