add date_range mapping type

[BeritJanssen]

Close #1292 .

[lukavdplas]

Looks good!

Can you add a screenshot of what this looks like in the document view?

[lukavdplas]

By the way, could you adjust your autoformatter to only format your changes rather than the whole file? If you want to format existing code, I would prefer a separate PR like #1356

[BeritJanssen]

This is how it now looks in the document overview for a testset from the peaceportal-epidat corpus:

NB: we still need to address #1257 before this becomes really useful for corpora with large date ranges. Since peaceportal-iis features BC data, PeacePortal is probably not the best candidate for implementing the date filter - but it did prove a nice proof of concept for the date_range type.