xml_func.py: Removed no longer used structural functions

mappings/for_review/DwC2-VegBIEN.specimens.fields.csv: input root: Removed DwC XML path info since DwC is now a CSV schema

mappings/DwC2-VegBIEN.specimens.csv: eventDate: Also map to obsstartdate/obsenddate, since the collectiondate is also the event date for specimens data, and for mergability with VegCSV

mappings/VegCSV-VegBIEN.specimens.csv: eventDate: Added mappings to obsstartdate/obsenddate, since users of this field (currently SALVIAS census_date) intend it as the plot event's date. Keep the mapping to collectiondate because a non-range plot event date is also the collectiondate of all organisms in that plot event.

schemas/py_functions.sql: parse_date_range(): Always return a value for end date, even if string is not a date range. This enables using _dateRangeEnd() as a filter function on anything intended as an end date.

mappings/DwC2-VegBIEN.specimens.csv, VegCSV-VegBIEN.specimens.csv: eventDate: collectiondate mapping: Removed _dateRangeStart filter because the eventDate (obsstartdate) is only valid as the date the specimen was collected if it is a single date, not a date range. (It is still valid as the obsstartdate/obsenddate if it's a range.)

mappings/Veg+.terms.csv: Added dateCollected

input via maps: Removed _date/date filter from date fields because the main mappings now have _date around all dates, so this filter is redundant

inputs/SALVIAS-CSV/maps/VegCSV.organisms.csv: census_date: Don't map directly to the year, as this field is allowed to be a full date even though our data sample contains only years. Note that _date/date will automatically detect plain years and treat them as years, and so will casts to timestamp.

inputs/SALVIAS*/maps/VegCSV.organisms.csv: census_date: Documented that this is for the subplot, not the organism, as all organisms in a subplot have the same value for it