Create date quality categories about gbif-dataset-metrics

[peterdesmet]

The pretty useless process if we just use GBIF issues:

IF issue CONTAINS (
        RECORDED_DATE_INVALID
        RECORDED_DATE_MISMATCH
        RECORDED_DATE_UNLIKELY
    )
   THEN category="Date with issues"
ELSEIF eventDate != ""
   THEN category="Valuable date (all in ISO8601)"
ELSE
   THEN category="Date not provided" /* This is just incorrect! See issue #27 */

[peterdesmet]

@bartaelterman, @niconoe, I need your feedback on this issue:

1. We need to look in verbatim.txt to get a useful eventDate (as GBIF overwrites them without warning in occurrence.txt, see #27 - need to confirm with them that no field in occurrence.txt has the original eventDate). If so, how challenging is it to loop over that file too?
2. Do we use the Canadensys Narwhal processor to provide high quality categories, instead of the current basic ones?
3. Or do we not tackle this issue in this POC?