Implementation of non uniform grid

The code can deal with stretched Cartesian mesh.
The _infoMesh file has to be adapted.
When the mesh is uniform, specify the number of cells.
When it is stretched, specify the stretched ratio.
Modify the _infoMesh after changing the code for stretched mesh.

Also add an example of the cylinder with stretched grid: 'cylinder_non_uniform'.
Cavity and cylinder remains with uniform grids.
Add arguments parser in scripts.

Add arguments parser using argparse.
Modifications in the post-processing files
and the script to generate a cylinder.
Type python 'script.py' --help to have the list of arguments.