restrucutre CAD examples

[shimwell]

change CAD examples to a slightly different layout

• task_15_making_dagmc_surface_mesh_geometry_from_cad
  • task_0_what_is_surface_mesh_geometry.py
  • task_1_converting_step_files.py
  • task_2_converting_cadquery_objects.py
  • task_3_converting_paramak_objects.py
• task_16_dagmc_cell_tally_fast_flux
  • task_1_simulation.py
• task_17_making_dagmc_unstructured_mesh_geometry_from_cad
  • task_1_converting_step_files.py
  • task_2_converting_cadquery_objects.py
• task_17_dagmc_unstrucutured_mesh_tally_heating
  • task_1_simulation.py