Create an example notebook for the CollisionFrequency class

[pheuer]

Create a tutorial notebook for the CollisionFrequency class that explains a lot of the physics behind this calculation. Some ideas:

1. Go through the calculation of the collision frequencies, talk about the fast and slow limits, include a plot of psi like this:
   

2. Talk about calculation of the mean free path from the collision frequency and its role as a metric of collisionality.

3. Talk about the various other collision frequencies available as attributes, e.g the Maxwellian-averaged frequencies (Replace fundamental collision frequencies with CollisionFrequencies #1699. Move fundamental collision frequencies to MaxwellianCollisionFrequencies #1703) and temperature isotropization and equilibration times (Add temperature isotropization and thermal equilibration to CollisionFrequencies  #1709)