bin/map: Prevent collisions if multiple inputs mapping to same output

mappings/DwC1-DwC2.specimens.csv: Mapped collectorNumber and recordNumber to recordNumber with _alt so they wouldn't collide when every input column, even empty ones, are created in the XML tree

bin/map: If out_is_db, in debug mode, print each row's XML tree and each value that it's putting

bin/map: If out_is_db, in debug mode, print the template XML tree used to insert a sample row into the DB

bin/map: map_table(): When translating mappings to column indexes, use appends to a new list instead of deletions from an existing list to simplify the algorithm

union: Omit mappings that are mapped to in the input map, in addition to mappings that were overridden. This prevents multiple outputs being created for both the renamed and original mappings, causing duplicate output nodes when one XML tree is used for all rows.

union: Omit mappings that are mapped to in the input map, in addition to mappings that were overridden. This prevents multiple outputs being created for both the renamed and original mappings, causing duplicate output nodes when one XML tree is used for all rows.

input.Makefile: Maps building: Via maps cleanup: subtract: Include comment column so commented mappings are never removed

subtract: Support "ragged rows" that have fewer columns than the specified column numbers

util.py: list_subset(): Added default param to specify the value to use for invalid indexes (if any)