Add cell2fate model with amortized inference

[AlexanderAivazidis]

More scalable cell2fate model including:

• amortized inference
• direct computation of 0.25, 0.5, 0.75 quantiles of the posterior
• corresponding tutorial notebook added to documentation
• unit tests to keep overall code coverage above 90%

[AlexanderAivazidis]

Just these two left to do:

• corresponding tutorial notebook added to documentation
• unit tests to keep overall code coverage above 90%