- change the constructor wrapper.
- in the new method, leave the pointer _c to NULL.
- in the init, allocate it.
- It seems ok to leave the object in this non initialized state,
but that is not so clear from the doc.
Added check for this pointer == NULL in converters.
When constructing the last unit vector in 2D, the sanity check was wrong because of usage of abs instead of std::abs.
Added method energy_on_bz_path_2 that returns the energy *matrix* at each k point on a given path instead of the eigenvalues of this matrix. The name of the function should be changed (to energy_matrix_on_bz_path?)
Renaming energies_on_bz_path_2 to energy_matrix_on_bz_path
