Added accessor positive only to MeshImFreq
Added linear_to_index to matsub_freq
Fixed abs bug in bravais_lattice + added method

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?)