CarbonNanotube-ExcitonEnergy: Low resolution exciton dispersion

This version is the same of the original algorithm. The resolution of exciton dispersion is the same as those of the basis function. In the next version we use a higher resolution for exciton dispersion compared to those of the basis function. Part of the code (interpolation of dielectric function and self-energy) that is used in the next version is implemented here.