Define spins at reference frequency for IMRPhenomP

- This patch uses f_ref instead of f_min in XLALSimIMRPhenomPCalculateModelParameters()
  to compute the IMRPhenomP model internal physical parameters from the LAL frame.
- The main PhenomP function XLALSimIMRPhenomP() also needs to take f_ref to compute
  integration constants for the alpha and epsilon angles. This is done so that for
  aligned-spin a change in f_ref should not change the phase of the waveform.
- Check whether f_ref is positive in the PhenomP functions. If f_ref = 0 is passed to
  ChooseFDWaveform then we set f_ref = f_min there.
- Change comment for phic parameter to be
  /**< Orbital phase at the peak of the underlying non precessing model (rad) */