Fix path to dielectric constant files in two-layered sphere test

  # External medium dielectric constant

  extern_diel : 1.000e+00

  # Dielectric constant files folder

  dielec_path : "."

  dielec_path : "../../ref_data"

  # List of dielectric constant files (used if diel_flag = 0)

  dielec_file : [ "eps_draine_Si.csv", "eps_ashok_C.csv"]

  # Dielectric constant files format (same for all files)