Document the `pandas` measurement sub-system

[goodboy]

Issue by tgoodlet Monday Dec 21, 2015 at 14:55 GMT Originally opened as https://github.com/sangoma/switchy/issues/21

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This includes docs for the Measurers collection type, DataStorer and how it interfaces with measurement apps, and the AppManager.

We probably need:

• example of how to write a measurement collection app
  • data storer insertion/api and app class attrs...
• pandas.DataFrame schema description and processing/analysis examples
• default figure walk-through with example
• operators api
• HDF storage and retrieval
• figspec manipulation / extension