refactor(Mesh): use the new Mesh API

What does this MR do?

• Refactor Mesh iterator to access values on a const variable.
• Use the new Mesh API in ::fwDataTools::Mesh to compute normals and improve unit tests.
  • Also fix the normals: when the cell contains more than 3 points, only the last 3 points were used to compute the normal.
• Improve Image ans Array documentation.

How to test it?

Launch the unit tests.

Launch Tuto14MeshGenerator and try to compute and shake normals.

Test the Mesh conversion using fwVtkIO: load a mesh, save a mesh, display it with VTK....

Associated Issues/Merge Requests

• Issues
  • Refs #420 (closed)