Add diagnostics using arviz to tutorials

[sbidari]

Two changes made:

• added mcmc diagnostics to the two tutorials
• changed num_chains=2 from deafult value of 1. This was required to suppress a warning about convergence diagnostics

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.47%. Comparing base (84f7809) to head (3e8cb63).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #219 +/- ## ======================================= Coverage 92.47% 92.47% ======================================= Files 40 40 Lines 877 877 ======================================= Hits 811 811 Misses 66 66 ``` | [Flag](https://app.codecov.io/gh/CDCgov/multisignal-epi-inference/pull/219/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CDCgov) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/CDCgov/multisignal-epi-inference/pull/219/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CDCgov) | `92.47% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CDCgov#carryforward-flags-in-the-pull-request-comment) to find out more.

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