Update Stochastic Volatility notebook to PyMC v5 (#517)

* Update notebook to pymc version 5

* rerun with pymc v5

* Rerun notebook with pymc v5

* Update GuassianRandomWalk

* Update Stochastic Volatility

* run sequentially

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Co-authored-by: SangamSwadiK <35230623+SangamSwadiK@users.noreply.github.com>
Co-authored-by: Oriol (ZBook) <oriol.abril.pla@gmail.com>

Author: 3 people