Docs and examples

[smartalecH]

Here a some important areas that could benefit from improved documentation and examples:

• Vector field formulations -- discuss the nuances of each method, how the loss function is computed, etc.
• Convergence -- wrt vector field formulation, number of terms, etc.
• Sharp bits example -- (e.g. what happens when the wavelength is a multiple of the unit cell...) all the gotchas
• Explanation of types, calculation basics
• Scattering matrix fundamentals
• Field processing routines
• Errata