class_docstr: Context manager that times the duration of the code body.
  The `duration` attribute will contain the execution time in seconds.
code: def __enter__(self):
      self.start = time.time()
      return self
  
imports: from __future__ import unicode_literals
  import time
label_desc: Use image node_id 2 for calling the TimeIt obj's underlying member method code with example usage: obj.__enter__() and returns: self