schemas/functions.sql: Removed no longer used join_strs_transform_fold_empty()

schemas/functions.sql: join_strs() aggregate: Use join_strs_transform_preserve_empty() as an optimization because all our data has already had '' replaced with NULL by sql_io.cleanup_table() in csv2db. This will help speed up _merges now that they are performed on a large scale in the slowest datasource, SpeciesLink.

bin/map: collision_suffix: Changed to use _merge instead of _alt to avoid losing source data on import when multiple fields collide

bin/map: Preventing collisions if multiple inputs mapping to same output: Made collision suffix configurable so it can easily be changed

bin/map: Preventing collisions if multiple inputs mapping to same output: Made collision suffix configurable so it can easily be changed

mappings/DwC2-VegBIEN.specimens.csv, VegCSV-VegBIEN.specimens.csv: taxonoccurrence.sourceaccessioncode mappings: Added catalogNumber mapping, which takes precendence over recordNumber and is applicable to specimens data and direct vouchers. recordNumber should only be used as a last resort (before the taxon name) because this is collector-assigned and often not unique within anything.

mappings/VegCSV-VegBIEN.specimens.csv: catalogNumber: Moved direct/indirect voucher _ifs inwards to wrap just the value of catalognumber_dwc, not the catalognumber_dwc field node, so that a future SQL function implementation of _if only needs to concern itself with returning one value or another, not with handling XML subtrees. The previous moving of the _ifs in r3942 was intended to effect this, but the _ifs weren't moved in far enough to wrap just the value.

mappings/VegCSV-VegBIEN.specimens.csv: eventDate mappings: Removed collectiondate mapping because the eventDate refers only to the plot event. Added /_alt suffixes for mergability with DwC.

mappings/DwC2-VegBIEN.specimens.csv, DwC1-DwC2.specimens.csv: Split eventDate into eventDate and dateCollected, where eventDate refers only to the date of the sampling event, but dateCollected also refers to the date the particular specimen was collected. (This distinction is important in merging with VegCSV, because in plots data, these two fields are distinct.) Remapped datasources with dateCollected-related fields to new dateCollected.

bin/map: Run new xml_func.simplify() on the root before printing the put template, so that _alts and _merges with only one element for the current datasource will be printed in their simplified form (with the _alt/_merge removed). This faciliates automated testing after an _alt/_merge suffix has been added, because the put template provided as part of the automated test will only change for those datasources that actually have an entry for both mappings, which greatly reduces the number of tests that need to be accepted.