add notebook: amortized point estimation

[vpratz]

A quick demonstration on how to use the new AmortizedPointEstimator class discussed in #121. Contains the illustrative example of the original paper.

[stefanradev93]

Very nice! The ConfigurableMLP is now part of the library in helper_networks.py, so the notebook can directly import and use it.

[vpratz]

Thanks for the review! I extended ConfigurableMLP to allow for arbitrary output dimensionality, so now we can use it for the current use case (input_dim!=output_dim) as well.

[stefanradev93]

Cool and good idea with the different dimensionalities!