Common headers for python script cells

[okennedy]

Current behavior: Each python cell gets a fresh interpreter and is prefixed with a hardcoded setup script (e.g., to initialize the vizierdb variable, bokeh, etc...).

Desired behavior: Allow users to provide custom setup code to be run in each cell (e.g., to avoid the mountain of from X import Y statements that will end up getting copied into each cell)

Topics for discussion:

• How does this setup code interact with dependencies? Presumably, changing this cell would require re-running ALL existing python cells.
• Maybe instead of a generic, turing-complete header, we simply let the user specify a list of desired includes. E.g., As part of the notebook configuration, we have a list of from X import Y. As long as there is no name collision, this should (I think) be side-effect free.