Add features to PlotSinglePosterior

[maho3]

Addresses #160

New features:

• Now can specify seaborn-like plot_kws when calling PlotSinglePosterior
• Now can overplot multiple posteriors from different models
• Adds functionality to specify lower and upper limits for subplots
• Default PlotSinglePosterior error bars are shown at central 68% and 95% percentile intervals.
• Also, moving 'Toy NPE Hyperparameter Search' in lampe tutorial to after the 'QM7b dataset', since it takes a while to run, and I assume people will blindly run these notebooks in serial.

Todo:

• [x] Update relevant tutorials
• [x] Add tests

[codecov-commenter]

:warning: Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.89%. Comparing base (26e189c) to head (c908e7e).

Files	Patch %	Lines
ili/validation/metrics.py	95.83%	1 Missing :warning:

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #163 +/- ## ========================================== - Coverage 96.91% 96.89% -0.02% ========================================== Files 23 23 Lines 1621 1644 +23 ========================================== + Hits 1571 1593 +22 - Misses 50 51 +1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.