Introduce nonlinearity more naturally

[DanielSank]

Section 3.1 goes from the linear to nonlinear case without warning. We need to state what we're doing there. There are two places in particular where we can do this:

1. "Now in the quantum mechanical case...on resonance with one particular transition ... and far off resonance from any other transition" We should explain that this condition requires anharmonicity.

2. Also in the introduction of the raising/lowering operator restriction to the 2-level space, we can be more explicit.