Document the `pandas` measurement sub-system

[goodboy]

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