Visualization module update

[Vaeliss]

Main contributions:

• heatmap plots now no longer require july library, hence matplotlib specific version is no longer required too.
• new weekday_barplot function to visualize an aggregate statistic (usually mean) of a desired metric across the different days of the week. Variability of such aggregate statistic can also be added in terms of std, iqr, or minmax range.
• added documentation to several viz functions
• plot_trend_analysis is now more general, a metric_fn of pywearable has to be specified with the usual parameters and trend analysis is performed. This required a refactor of the function and some small changes for compatibility with sleep summary function.
• new longitudinal chart to show at the same time sleep, cardiac, and activity data for a user throughout a period of interest, divided in 24 hours chunks, shown in different subplots. This function is for now experimental and will require some small updates once every loader will have the same behaviour for the loading of hypnograms, heart rate, and (more importantly since now it isn't at all the case) steps series.

[Vaeliss]

Need to rebase this with new recent updates to sleep module, in order to understand what breaks down. Then I will commit the changes needed in order to make everything work again. Wait to accept the pull request until further notice

[Vaeliss]

Rebase on top of sleep module update has been done. @dado93, when you get the chance take a look if everything looks good, then we proceed with the merge