code: def init_default_headers(self):
          
          if not self.data_model:
              self.data_model = validate_upload.get_data_model()
              if not self.data_model:
                  print("Can't access MagIC-data-model at the moment.\nIf you are working offline, make sure MagIC-data-model.txt is in your PmagPy directory (or download it from https://github.com/ltauxe/PmagPy and put it in your PmagPy directory).\nOtherwise, check your internet connection")
                  return False
  
          # actual is at position 0, reqd is at position 1, optional at position 2
          self.headers['measurement']['er'][1], self.headers['measurement']['er'][2] = self.get_headers('magic_measurements')
          self.headers['specimen']['er'][1], self.headers['specimen']['er'][2] = self.get_headers('er_specimens')
          self.headers['sample']['er'][1], self.headers['sample']['er'][2] = self.get_headers('er_samples')
          self.headers['site']['er'][1], self.headers['site']['er'][2] = self.get_headers('er_sites')
          self.headers['location']['er'][1], self.headers['location']['er'][2] = self.get_headers('er_locations')
          self.headers['age']['er'][1], self.headers['age']['er'][2] = self.get_headers('er_ages')
  
          self.headers['result']['pmag'][1], self.headers['result']['pmag'][2] = self.get_headers('pmag_results')
          self.headers['specimen']['pmag'][1], self.headers['specimen']['pmag'][2] = self.get_headers('pmag_specimens')
          self.headers['sample']['pmag'][1], self.headers['sample']['pmag'][2] = self.get_headers('pmag_samples')
          self.headers['site']['pmag'][1], self.headers['site']['pmag'][2] = self.get_headers('pmag_sites')
docs: initialize default required headers.
          if there were any pre-existing headers, keep them also.