Add 2D Vortex example to notebooks

This is adding a new 2D vortex example to PyClaw notebooks. This is example 2.2 in the paper https://www.global-sci.org/v1/cicp/issue/FULLPDF/9/807/paper.pdf?code=5PGikBxEljgnVQwsaSy7Hg%3D%3D . It demonstrates how to solve the 2D Euler equations for compressible gas dynamics with periodic boundary conditions and perturbations to the mean flow. This notebook compares the performance between classical Clawpack and SharpClaw WENO solvers. The problem is initiated in the issue clawpack/pyclaw#714.