rewrite radon notebook using ArviZ and xarray (#3963)

* rewrite radon notebook using ArviZ and xarray

Roughly half notebook has been updated

* add comments on xarray usage

* rewrite 2n half of notebook

* minor fix

* rerun notebook and minor changes

* rerun notebook on pymc3.9.2 and ArviZ 0.9.0

* remove unused import

* add change to release notes

Author: OriolAbril

RELEASE-NOTES.md

@@ -1,13 +1,14 @@
 # Release Notes
 
 ## PyMC3 3.9.x (on deck)
-*waiting for contributions*
+### Documentation
+- Notebook on [multilevel modeling](https://docs.pymc.io/notebooks/multilevel_modeling.html) has been rewritten to showcase ArviZ and xarray usage for inference result analysis (see [#3963](https://github.com/pymc-devs/pymc3/pull/3963))
 
 ## PyMC3 3.9.2 (24 June 2020)
 ### Maintenance
 - Warning added in GP module when `input_dim` is lower than the number of columns in `X` to compute the covariance function (see [#3974](https://github.com/pymc-devs/pymc3/pull/3974)).
 - Pass the `tune` argument from `sample` when using `advi+adapt_diag_grad` (see issue [#3965](https://github.com/pymc-devs/pymc3/issues/3965), fixed by [#3979](https://github.com/pymc-devs/pymc3/pull/3979)).
-- Add simple test case for new coords and dims feature in `pm.Model` (see [#3977](https://github.com/pymc-devs/pymc3/pull/3977)). 
+- Add simple test case for new coords and dims feature in `pm.Model` (see [#3977](https://github.com/pymc-devs/pymc3/pull/3977)).
 - Require ArviZ >= 0.9.0 (see [#3977](https://github.com/pymc-devs/pymc3/pull/3977)).
 
 _NB: The `docs/*` folder is still removed from the tarball due to an upload size limit on PyPi._