Create an eml.xml in each DwC-A download

[dshorthouse]

Custom downloads of datasets in the Explorer should include at the very least, key/value pairs of the filters used to construct the dataset. Ideally, an eml.xml should be present in the download and be as fully populated as possible (eg geographicCoverage is populated when a geospatial filter is created, taxonomicCoverage is populated when there's a filter on taxa, alternateIdentifier is the URL, etc.). Because there's no appropriate place in the eml.xml for the key/value pairs of filters, I suggest these be added to the abstract element.