Switch to Vega as statistical visualisation library

[mwigham]

Tasks:

• Decide if we want to do this
• Work out some conventions - how will we organise the specs, how do we organise the data updates code, are there particular choices we prefer in the vega spec, colours etc.?
• Convert all existing stat visualisations to Vega where possible (including in showcases)
• Write documentation for data.beeldengeluid.nl developers with a link to vega tutorials and an explanation of the conventionsn we have chosen.