docstr: Number of frames.
code: def num_frames(self):
      """Number of frames."""
      return len(self)
  
imports: from __future__ import absolute_import, division, print_function
  import warnings
  import numpy
  import scipy.fftpack
  from ..processors import Processor
  from .signal import Signal, FramedSignal
label_desc: Use image node_id 1 for calling the _PropertyMixin obj's underlying member method code with example usage: obj.num_frames() and returns: len