docs: def trajectory(self, ini, end, delta, mass_coo, age_in_sec=False,
                     online=False):
          """
          create a trajectory out of a stellar model
  
          Parameters
          ----------
          ini : integer
              Initial model, inital cycle number.
          end : integer
              Final model, final cycle number.
          delta : integer
              Sparsity factor of the frames.
          mass_coo : float
              Mass coordinate for the traj.
          age_in_sec : boolean, optional
              Set to True if age in se file is in seconds (like in MESA).
              The default is False.
  
          Returns
          --------
          float
              radius_at_mass_coo, density_at_mass_coo,
              temperature_at_mass_coo, age_all
  
          Notes
          -----
          plus writes a file with the trajectory information to be used
          with ppn.
  
          Warning: remove the old trajectory, if you have any for the same
          mass coordinate.  You are appending data, not overwriting.
  
          Update: this method works for output types with indexes going
          from the outside in (MESA) or the other way around.  Also the
          requested quantities are linearly interpolated in the mass
          shell.
          online: boolean, optional
          are you working online in the ipython notebook? If so,
          you will be given an HTML link to download the file.
  
          """
  
          filename='traj_'+str(mass_coo)+'.dat'
          f = open(filename,'a')
          radius_at_mass_coo=[]
          density_at_mass_coo=[]
          temperature_at_mass_coo=[]
          masses=self.se.get(list(range(ini,end+1,delta)),'mass')
          temps=self.se.get(list(range(ini,end+1,delta)),'temperature')
          rhos=self.se.get(list(range(ini,end+1,delta)),'rho')
          radii=self.se.get(list(range(ini,end+1,delta)),'radius')
          ages=self.se.get(list(range(ini,end+1,delta)),'age')
          cycs=list(range(ini,end+1,delta))
          age_all=[]
          for i in range(len(ages)):
              age=ages[i]
              if age_in_sec:
                  age /= constants.one_year
              mass=masses[i]
              temperature=temps[i]
              rho=rhos[i]
              radius=radii[i]
              my_things=[temperature,rho,radius]
  
              if mass[0]>mass[len(mass)-1]:
                  zone_above=where(mass>mass_coo)[0][-1]
                  zone_below=zone_above+1
              else:
                  zone_above=where(mass>mass_coo)[0][0]
                  zone_below=zone_above-1
  
              if mass[zone_below]>mass[zone_above]:
                  sys.exit("ERROR: finding of zone index confused")
              all_things_interplt=[]
              for thing in my_things:
                  thing_interplt=thing[zone_below]+(mass_coo-mass[zone_below])* \
                      (thing[zone_above]-thing[zone_below])/(mass[zone_above]-mass[zone_below])
                  all_things_interplt.append(thing_interplt)
              this_temperature,this_rho,this_radius=all_things_interplt
  
              string = str(cycs[i])+'  '+str(age)+'  '+str(this_temperature)+'  '+str(this_rho)
              f.write(string+"\n")
              radius_at_mass_coo.append(this_radius)
              density_at_mass_coo.append(this_rho)
              temperature_at_mass_coo.append(this_temperature)
              age_all.append(age)
          f.close()
          if online:
              return FileLink(filename)
  
          return radius_at_mass_coo, density_at_mass_coo, temperature_at_mass_coo, age_all