class_docstr: Module wrapping a function provided by the user.
  Analogous to snt.Module
code: def layer_op(self, *args, **kwargs):
      return self._layer_op(*args, **kwargs)
  
imports: from __future__ import absolute_import, print_function
  from abc import ABCMeta, abstractmethod
  import numpy
  import tensorflow
  from six import with_metaclass
  from niftynet.engine.application_variables import RESTORABLE
label_desc: Use image node_id 2 for calling the LayerFromCallable obj's underlying member method code with example usage: obj.layer_op() and returns: self