allow morpho to detect online data and import it
In several situations (such as Bug# 1982) an EML description might have a URL
pointing at an actual data file that is referenced in the top level EML
distribution section. When this occurs, Morpho should be able to import the
data from the URL, asking for the entity and attribute metadata, and move the
distribution reference to the entity distribution section.
If the data are stored in Metacat, then they need not be uploaded again, but
rather a simple reference to the Metacat id needs to be made to associate the
new metadata with the existing data.
If the data are at some other online URL, the data may need to be stored in
metacat (but only after the user has confirmed the operation).