Feat/plotly express

[pradyot-09]

• The histograms, heatmaps and comparisons have been replaced with interactive Plotly graphs. Plotly.js is used to build the graphs on the go from json.
• Initial tests show that plotly reports are smaller in size compared to matplotlib and the takes way less time for report generation compared to matplotlib.
• use parameter 'online_report' to use plotly.js from cdn server and use report online. Else, plotly.js is embedded in the report and can be used offline too.

closes #164

[sbrugman]

Quick wins

• Substantial part of the data for the plotly javascript is the layout. This is the same for the plots. Reusing the same layout reduced the total file size by 50% in my experiments. Templates may be used as best practise: https://plotly.com/python/templates/#saving-and-distributing-custom-themes.
• Reports are slow to render. This can be solved by lazy loading the rendering of the plots (visible only, or one feature at a time). One example solution is using jQuery https://github.com/morr/jquery.appear, but many similar options exist. Feature-wise, we could leverage the custom-js https://github.com/ing-bank/popmon/blob/master/popmon/visualization/templates/assets/js/custom-script.js.

Improvements

• The layout above should reflect a consistent style that is readable and clear (be sure to use the same yellow/red, which differs from the default).
• The hover text for the histograms are difficult to read due to the overlay. Use unified hover mode: https://plotly.com/python/hover-text-and-formatting/#unified-hovermode
• The heatmaps have little padding. The aspect-ratio could be set to square: https://plotly.com/python/heatmaps/#controlling-aspect-ratio, and/or padding should be added
• The toolbar for the plots should be simplified by removing redundant icons :https://stackoverflow.com/questions/56659593/how-to-remove-icons-in-plotly-dash-chart-tool-bar/56659783#56659783. At least the logo and lasso tool can be removed.

Follow-up features:

• By default, the plotly express JS could be integrally included in the HTML file. The CDN option could be a new feature, as bootstrap could also be hosted by a third party. (the online_report option)
• Cross filter functionality would be awesome, where selecting a time-window on one plot also updates the others, e.g. https://plotly.github.io/plotly.js-crossfilter.js/

[sbrugman]

Closing in favour of #225