def append(other,axis:nil)
      other = self.class.cast(other)
      if axis
        if ndim != other.ndim
          raise DimensionError, "dimension mismatch"
        end
        return concatenate(other,axis:axis)
      else
        a = self.class.zeros(size+other.size)
        a[0...size] = self[true]
        a[size..-1] = other[true]
        return a
      end
    end
def append(other,axis:nil)
      other = self.class.cast(other)
      if axis
        if ndim != other.ndim
          raise DimensionError, "dimension mismatch"
        end
        return concatenate(other,axis:axis)
      else
        a = self.class.zeros(size+other.size)
        a[0...size] = self[true]
        a[size..-1] = other[true]
        return a
      end
    end
Append values to the end of an narray.
 @example
   a = Numo::DFloat[1, 2, 3]
   p a.append([[4, 5, 6], [7, 8, 9]])
   # Numo::DFloat#shape=[9]
   # [1, 2, 3, 4, 5, 6, 7, 8, 9]

   a = Numo::DFloat[[1, 2, 3]]
   p a.append([[4, 5, 6], [7, 8, 9]],axis:0)
   # Numo::DFloat#shape=[3,3]
   # [[1, 2, 3],
   #  [4, 5, 6],
   #  [7, 8, 9]]

   a = Numo::DFloat[[1, 2, 3], [4, 5, 6]]
   p a.append([7, 8, 9], axis:0)
   # in `append': dimension mismatch (Numo::NArray::DimensionError)