lag_energy_spectrum: Add outfile, lc_ref qualifier

* Via the lc_ref qualifier one can use a broad lightcurve as reference
  band. If not set, the default is to calculate the reference
  lightcurve as the sum of all energy-resolved lightcurves but the
  current channel-of-interest.
* Add outfile qualifier: If set, a FITS file is written with
  information about CPD, phase, averaging, etc.