ENH add h npe

[tomMoral]

What does this implement/fix? Explain your changes

Implement hNPE, from this paper: https://arxiv.org/abs/2102.06477
This PR adds:

• a hierachical_simulator decorator to wrap simulator and generate simulation with extra observations.
• a HIerarchicalDensityEstimator which factorizes the posterior and chains two flows to estimate global and local parameters
• a ``hierarchical_nnhelper to build a flow fromfactory`.

This PR also add an example of use case for this method.

Any relevant code examples, logs, error output, etc?

From the example:
Without hierarchical observation, with a model $x = \theta_1 \theta_2$, we get ill defined posterior:

With 10 extra observation which shares the same $\theta_2$, we get:

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[janfb]

Is this ready for review?