Suggest converting OSM approximate date to EDTF

[1ec5]

Detect an OSM approximate date pattern and suggest a fix to replace it with an EDTF tag.

Fixes OpenHistoricalMap/issues#547 and fixes OpenHistoricalMap/issues#657. Depends on #187.

[1ec5]

Based on OpenHistoricalMap/issues#547 (comment), I’ve been going with the set notation in EDTF level 2 instead of the extended interval notation in EDTF level 1. However, EDTF.js rejects qualifiers in sets, which prevents us from translating early/mid/late modifiers with approximation qualifiers. Academic systems such as the Bayou City DAMS apparently rely on approximation in intervals for this purpose.

[1ec5]

This is ready for review. I’ve added conversions for all the patterns listed in this documentation with the exception of the j: and jd: namespaces for Julian dates and day numbers. I’ll split out the calendrical notations as a separate task, since we may want to coin a new key to store the original date as specified in the source.

Between this documentation and the original proposal, there are a number of underspecified edge cases, which I’ve left out of the conversion code for simplicity. The openstreetmap-date-parser is somewhat more robust in the combinations of notations that it supports. Down the line, I’d like to integrate this functionality with that library (as long as @plepe is open to it) instead of maintaining the conversions here. That will benefit OSM as well as OHM.