OCSADV-154 target analysis

This is kind of random. I needed to write a little program to gather some stats on the target types we were using, and in the process relocated the `edu.gemini.shared.util.immutable` ↔︎ Scala conversion syntax and added another `parseDocument` overload to `PioSpXmlParser`. 

Just to confirm, this needs to go into the **develop** branch; it's not for December.
Oh, perhaps, but in a case like that you're better off with an implicit class ... I just have those defs above as stubs for code that's using the old implicit API; most users will just import `ScalaConverters._` (see below).