Added some helper functions to the swirl classes

Signed-off-by: Luca Clementi <luca.clementi@gmail.com>
Added the getDotFile function to create a dot file

getDotFile return a dot file to be used with the dot application
from graphviz package.
The graph gives a visual representation of the swirl

Signed-off-by: Luca Clementi <luca.clementi@gmail.com>
add the -g option to create a dot file

Signed-off-by: Luca Clementi <luca.clementi@gmail.com>