Graceful handling of Driver Front-end connections

[qwofford]

To see an example of this problem take a look at how this Parquet front-end is implemented:

dsi/dsi/drivers/parquet.py

Lines 79 to 80 in 6ecaa35

	print('Opening Jupyter notebook...')
	self.get_cmd_output(cmd=['jupyter-lab ./dsi_parquet_driver_output.ipynb'])

When this is called from an interactive terminal, you can cancel with control+c, but the spawned jupyter notebook remains.

This should be handled gracefully. The easy case is when a human is working interactively and then enters the control+c key command, this should be quick to implement.

The harder case is when an external program needs to signal to to the front-end that its work is done and the process should be terminated gracefully. To make this concrete, consider this example:

1. Greg writes a script which instantiates a DSI Core Terminal, but before the Terminal performs a transload operation (therefor locking the metadata schema), Greg needs a ML program to explore SQL data and return a sample of required data.
2. The Terminal loads a front-end SQLite Driver and an arbitrary back-end Driver.
3. The method "Terminal().artifact_handler(interaction_type='inspect')" is called.
4. The external ML program needs to execute SQL queries according to its own logic, and when the external program terminates it should send a signal to the DSI Core Terminal.
5. The SQL query results should then be available to guide how/which Plugins are loaded.