Design: Abstracting File Access

[denised]

To support both the Open Collaboratory work and (probably) some of the integration scenarios, we need an ability to have the system load data from someplace other than the json/csv files in the file system.

The abstraction must support loading from the local file system, but should also allow a client to pass in a handler to access data from another system instead.

This ticket is for designing that mechanism, in coordination with the Colab.

[denised]

I'm removing this from the "must have" features for Push to convert Excel to Python, since we've adopted a more limited scope approach to #401. But I'm leaving the issue as the starting point for a more general solution to that problem

[denised]

Closing as a dup of #401