Add pytem run command and basic notebook example

[louispt1]

Context

There is a desire to have a easier way to manage scenarios with minimal installation and knowledge requirements about PyETM for less tech savy users or situations where there is no need to do anything other than manage scenarios with the from_excel+to_excel approach.

Implemented changes

Added pyetm run <excel_file> command which completes a round trip to the ETM based on the settings in the input excel and the export config tab. This allows users who are less comfortable with code editors and scripting to avoid interacting with notebooks etc at all.

There is also a basic Jupyter notebook that demonstrates this round trip functionality, with some minor additional details that demonstrate how to inspect the scenarios object.

Checklist

• I have tested these changes
• I have updated documentation as needed
• I have tagged the relevant people for review