Add introduction

Text: 
- electric fields basics
- approximating electric fields
- incorporate information in domains

- interfaces
- charge transfer
- models
- linking to boundary conditions

- biophysics
- cable models
- HH dynamics
- myelinated models
- simplified measures

- optimisation

- link to implementation

Figures:
- circuit: interface
- circuit: cable
- circuit: HH dynamics
- circuit: myelinated model?
