LinearNetwork class and notebook example

[arvoelke]

Also include example of how to scale the radii based on the H2-norms for arbitrarily filtered white-noise input. Related to #6.

[arvoelke]

Have a way to automatically determine the radii based on H2-norm (with arbitrarily chosen filter on input), and/or the methods from On computing the worst-case norm of linear systems subject to inputs with magnitude bound and rate limit.

[arvoelke]

After #26, missing:

• notebook example
• radii support (with different "solvers")

[arvoelke]

Basic support and documentation completed by #41.