VegX mappings: Changed taxonDetermination of role identifier to instead have explicitly no role, because data providers' VegX files generally do not provide role information and we don't want the default taxonDetermination XPaths to require this

inputs/CTFS/maps/VegX.organisms.csv: Connected plot to plotObservation by using new support for backward (child-to-parent) pointers whose target is a text element containing an ID

xml_dom.py: get_id(): If the node doesn't have an ID, assumes the node itself is the ID. This enables backward (child-to-parent) pointers whose target is a text element containing an ID, rather than a regular element with an ID attribute.

VegX mappings: Map locationevent.sourceaccessioncode to plotUniqueIdentifier since this field is no longer being used by authorlocationcode

VegX mappings: Map the authorlocationcode to plotName instead of plotUniqueIdentifier because it's a better fit

inputs/CTFS/maps/VegX.organisms.csv: Fixed bug in Species taxonConcept mapping where the role was computer instead of identifier

xml_dom.py: value(): Skip comment nodes. This fixes a bug where comments inside text elements would prevent the value from being retrieved.

inputs/CTFS/test: Accepted test outputs for new VegX_CTFS_row_120000_bci.0.test.organisms.xml instead of VegX_CTFS_row_180000.0.test.organisms.xml, which didn't have <taxonNameUsageConcepts> that match up with <individualOrganisms>

inputs/CTFS/test: Accepted test outputs for new VegX_CTFS_row_120000_bci.0.test.organisms.xml instead of VegX_CTFS_row_180000.0.test.organisms.xml, which didn't have <taxonNameUsageConcepts> that match up with <individualOrganisms>

inputs/CTFS/maps/VegX.organisms.csv: Added taxonConcept mappings