Modified 'transverse_tracker.py' to correctly segment 1-turn integrated
quantities (like eg. app_x,y or Qp_x,y). General cleaning.
Fixes bug in _set_slice_index_of_particle() when using 'const_charge'.

'const_charge' and 'const_space' use different indexing
conventions for first_index_in_bin -> Use z_index attribute
instead for slice_index_of_particle.
Extends 'transverse_tracker.py'.

- Adds support for octupole cross terms app_xy, app_yx.
- In 'detune' method:
    - Adds effect of octupole cross terms app_xy, app_yx.
    - Makes implicit dependence of octupole parameters on p0 explicit.
    - Adapts definition of Jx, Jy.
- Adds convenience function to get octupole parameters for LHC from magnet
currents.