code: def copyto(self, src, where=None):
          
          for k in self.containers:
              v = self.__dict__[k]
              if isinstance(v, np.ndarray):
                  np.copyto(v, src.__dict__[k], where=where)
              else:
                  v.copyto(src.__dict__[k], where=where)
docs: Emulates function `copyto` in NumPy.
  
         Parameters
         ----------
  
         where: (N,) bool ndarray
              True if particle n in src must be copied. 
         src: (N,) `ThetaParticles` object 
              source 
  
         for each n such that where[n] is True, copy particle n in src
         into self (at location n)