replace mnle tutorial with iid-inference tutorial.

[janfb]

This PR replaces the MNLE tutorial with a new, more general tutorial about SBI with iid-data. This new tutorial also shows how to do iid-inference using SNPE, i.e., using permutation-invariant embedding nets.

The MNLE tutorial will be updated and extended to cover inference with experimental conditions, see #829

[codecov]

Codecov Report

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unittests	74.61% <ø> (ø)

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[michaeldeistler]

I did not read through the whole tutorial, but I like the idea of having a separate tutorial for iid data. Thanks for tackling this!