* publish parameters
** problem for now is that PairTersoff stores parameters as an n*n
   array of struct
** KIM uses an n*n array for every parameter
** Therefore store all the array pointers in a struct.
** This means the following modifications need to be done:
*** model_driver_init: get free and fixed parameters and store the
   pointers in PairTersoff, register "reinit" function
*** reinit: re-compute derived paramenters (fixed parameters), like
   for example the cutoff, c²/d² etc. Do this by calling
   PairTersoff::prepare_params.
*** PairTersoff::PairTersoff: get pointers from model_driver_init and
   store them, call prepare_params
*** PairTersoff::read_params: handle new storage layout for
   parameters
*** PairTersoff::prepare_params: handle new storage layout for
   parameters
*** PairTersoff::compute: handle new storage layout for
   parameters
** Benchmark if that impacts the performance.

* investigate if there is a way to support half neighbor lists without
   getting terrible performance

* optimize
** parts of force calculation are still done even if not needed

* test suite

* comment code better

* use assert() macro to check bounds etc and disable for release builds