class_docstr: Class that creates a callable object that uses mouse clicks or key
  clicks on a figure window to place contour labels.
code: def __init__(self, cs):
      self.cs = cs
      BlockingMouseInput.__init__(self, fig=cs.ax.figure)
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import six
  import matplotlib.lines
  import logging
label_desc: Use image node_id 1 to create a new BlockingContourLabeler object from inherited base classes: BlockingMouseInput with example: obj = BlockingContourLabeler(cs)