Suggestions to improve practicals

• add a chunk to draw all MA plots in an array - fixed in commit 19d8b803
• move df_raw definition later in the script - fixed in commit 68712c49
• remove manual computation of counts in UQ - fixed in commit 1645344b
• add a gglotly plot of MA and volcano - already done (somehow)
• add labels in PCA for ggplotly (when hovering) - complicated because not directly handled with ggplot