ph5tostationxml - only add a network, station, or channel to the inventory if it hasn't already been added

[nick-falco]

I found a bug in the current ph5tostationxml implementation where if you supply a list of duplicate requests it adds them to the inventory more than one time. This causes the program to produce StationXML containing duplicates. The good news is that for the command line version of the program this shouldn't have affected anything since only one request gets processed at a time, but I had to revert the web services back to the stable v4.0.2.

We need to add a check in ph5tostationxml to only add a network, station, or channel to the inventory if it hasn't already been added.