Various small fixes

Fixed z_max output, wiped some unused code...
Changed integration method, fixed field

- Changed integration method from "perturbation" to simple. This vastly
  decreases integration errors.
- Updated field calculation. Now using linear by z approximation of cell
  field.
- Fixed a typo in RK2 method: mixed up k1_r and k1_v in second group
  coefficient formulas.
- Updated documentation.