VAE deprecation warning from tf.distributions

[tsrobinson]

Running MIDAS using VAE leads to deprecation warning re. tf.compat.v1.distributions.

E.g.

>>> tf.compat.v1.distributions.Normal()
WARNING:tensorflow:From <stdin>:1: Normal.__init__ (from tensorflow.python.ops.distributions.normal) is deprecated and will be removed after 2019-01-01.
Instructions for updating:
The TensorFlow Distributions library has moved to TensorFlow Probability (https://github.com/tensorflow/probability). You should update all references to use `tfp.distributions` instead of `tf.distributions`.

Migrating affected code to tfp.distributions is not straightforward as not designed for TF1 graph-oriented model. We should investigate solutions to safeguard codebase in medium term.