WIP: bugfixes and enhancements to objectives

[gabgoh]

New formatting for objectives

>>> a   = f(1)
>>> b   = f(2)
>>> c   = f(3)

>>> a                                                                                                
F(1)

>>> b                                                                                                
F(2)

>>> a + 2*b                                                                                               
(F(1) + F(2)·2·-1)

Access to tensors in after objectives is available now

>>> z = a + 2*b   
>>> z(T)
>>> a.value                                                                                            
 <tf.Tensor 'Mean:0' shape=() dtype=float32>

TODO: Realize is a potentially breaking change to the lower levels of the lucid API. Make sure Ludwig is ok with the changes and all unit tests pass.

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[ludwigschubert]

Gabe, please rebase on master@HEAD—I've fixed an unrelated issue with Python 2 compatibility that didn't even let you run the (now still failing) tests in the Python 2 env.

[gabgoh]

@googlebot I signed it!

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CLAs look good, thanks!

[ludwigschubert]

@gabgoh I'd still happily merge this, but I need a little bit of help. Can you rebase on tensorflow/lucid HEAD? Or transfer the branch from your fork to this repository so I can do so myself?