Notebook to demonstrate advanced usage such as combining GP with Neur…

…al Network (GPflow#712)

* added notebook

* added vanilla conv net accuracy

* Tidy up notebook

* ran notebook

* ran notebook GPflow#2

* add untested notebooks to test_notebooks

* fix bug in natural_gradients notebook

* add nb_niter and nb_range to only run a small number of iterations in continuous integration notebook tests

* make advanced_usage.ipynb use nb_iter()

* don't add natural_gradients.ipynb and upper_bound.ipynb notebook tests in this PR

* make advanced_usage run faster in CI

* clean up advanced_usage

* Update test_notebooks.py

* fix notebook_niter

* missing input_dim for CI

* update notebook

* another reset_default_graph_and_session call