Workshop ready

Summary

• Add df_to_numeric() to easily format data frames.
• Add plot_feature_normalized_data() to plot normalized sample-level data for a single feature.
• Add plot_feature_logfc() to plot differential analysis results for a single feature.
• Add more examples and screenshots to the "Get Started" vignette.

What's Changed

• plot single feature by @nicolerg in #35

Full Changelog: v1.2.0...v1.3.0