Link PMT digits to hits

Recover the link between pmt digits (adc, tdc) and the hits.

current structure:

struct pe
{
  double time;
  int h_index;
};

struct dg_ps
{
  int side;
  std::vector<double> adc;
  std::vector<double> tdc;
  std::vector<pe> photo_el;
};

struct dg_cell
{
  int id;
  double z;
  double y;
  double x;
  double l;
  int mod;
  int lay;
  int cel;
  dg_ps ps1;
  dg_ps ps2;
};

proposed structure:

struct pe
{
  double time;
  int h_index;
};

// photo-signal

struct dg_ps
{
  double adc;
  double tdc;
  std::vector<pe> photo_el;
};

struct dg_cell
{
  int id;
  double z;
  double y;
  double x;
  double l;
  int mod;
  int lay;
  int cel;
  std::vector<dg_ps> ps1;
  std::vector<dg_ps> ps2;
};