Add convenience methods and new warning to DocumentRouter

[jklynch]

This PR proposes two modifications to DocumentRouter:

• deliver a warning if a DocumentRouter processes the same start document twice consecutively to address #164

• add convenience methods to address #165

  • get_start() returning the most recently processed start document
  • get_descriptor(event_doc) returning the descriptor document corresponding to the specified event document
  • get_stream_name(event_doc) returning the stream name for the specified event document; this seems like a minor convenience but it is the piece of information I very often want from a descriptor

The convenience methods are prefixed with get_ since DocumentRouter already defines start(name, doc) and descriptor(name, doc) but some other naming scheme might be better.

I realize now that the event_doc parameter for get_descriptor and get_stream_name should probably just be doc.

[tacaswell]

Needs tests and docstrings, but I'm :+1: on this.

This is very common book keeping and in the grand scheme of things relatively cheap. I'm still meh on the stream name helper, but won't argue over it.

[jklynch]

The latest commits have a proposal for a minimal change to RunRouter. The warning is still issued but the exception is re-raised.