ISIS Serial Number translation only uses the last translation

In the ISIS serial number translation logic, the SQLAlchemy model converts the PVL translation string into a [NextedJsonObject](https://github.com/USGS-Astrogeology/plio/blob/master/plio/io/io_db.py#L37) which removes duplicate keys such as the multiple translations. Then, the JSON object gets turned back into a PvlModule. This results in strange behavior like the Galileo translation going from:

```
Group = Keyword1
Auto
InputKey = SpacecraftName
InputGroup = "IsisCube,Instrument"
InputPosition = (IsisCube, Instrument)
OutputName = Keyword1
OutputPosition = (Group, SerialNumberKeywords)
Translation = (Galileo, GalileoOrbiter)
Translation = (Galileo, GALILEOORBITER)
Translation = (Galileo, "Galileo Orbiter")
Translation = (*, *)
End_Group
```

to

```
Group = Keyword1
Auto
InputKey = SpacecraftName
InputGroup = "IsisCube,Instrument"
InputPosition = (IsisCube, Instrument)
OutputName = Keyword1
OutputPosition = (Group, SerialNumberKeywords)
Translation = (*, *)
End_Group
```

This in turn results in invalid serial numbers because the value on the label is "Galileo Orbiter" and instead of getting "Galileo" as the first keyword, you get "Galileo Orbiter".