Expand vignette with more examples

[matt-graham]

Resolves #21 and resolves #7.

Expands the vignette with more examples including an illustration of using posterior package to compute summary statistics and diagnostics, which now that the traces value returned by sample_chain is a matrix with named columns corresponding to variables and rows chain iterations (changed from previous dataframe in #33), can be achieved by directly passing the traces matrix to the summarize_draws function in the posterior package (or first converting to a draws object and then using the summary generic).

The vignette now compares adaptation using both the Barker and Langevin proposals to highlight the more robust and stable behaviour of the latter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (29cedd4) to head (e0a75d9).
Report is 21 commits behind head on main.

Additional details and impacted files

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