velocity_to_eps
 
  eps_r is the relative dieletric permittivity
  v is the velocity of the phase (m/ns)
 
  sig is the eletrical conductivity measured in mS/m
  f is the frequency. If f is set to 0 a high frequency approxiamtion is
  applied
 
  (C) Knud Cordua, 2016, Thomas Mejer Hansen, 2016