lag_energy_spectrum: add deadtime qual., CPD ascii output, verbosity handling

- Parse deadtime qualifier into foucalc
- Better verbosity handling
- Add option to write cross spectra (frequency versus imaginary/real
  part of cross power density) into an ASCII file. These spectra can
  be converted into a PHA file (-> ftrbnrmf) and fitted with the
  Reltrans model. Currently disabled.