class_docstr: Module wrapping a function provided by the user.
  Analogous to snt.Module
code: def __init__(self, layer_op, name="from_callable_op"):
      super(LayerFromCallable, self).__init__(name=name)
      if not callable(layer_op):
          tf.logging.fatal("layer_op must be callable.")
          raise TypeError
      self._layer_op = layer_op
  
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 1 to create a new LayerFromCallable object from inherited base classes: Layer with example: obj = LayerFromCallable(layer_op, name)