lag_energy_spectrum: Improve run-time for freq-resolved LES

* Previously, frequency-resolved lag-energy spectrum could be
  completed by looping over the frequency bins and then calling
  lag_energy_spectrum, which loops through the energy bins. However,
  it is much faster to first loop through the energies and then
  through the frequencies (the DFT has to be done only once for each
  LC).
* Change f_min, f_max qualifiers to be arrays (containing the
  frequency grid) instead of doubles. The loop through the frequency
  bins is now implemented in the lag_energy_spectrum function
* Change output struct: now contains all available information. This
  includes a change of the field errlag -> errlag_nowak99
* Export file writing into own function, remove option to pass open
  file pointer to lag_energy_spectrum function