Add an option to start over

Currently, when the application starts, the user is prompted to upload the data. The application then proceeds to develop ML code based on user's selections from the UI. If the user wants to start over, they can upload new data, but the generated code continues to grow: it includes the previous data and user's selections and the new ones. It would be nicer to have a clean "start over" option, such that if the user messed up and wants a redo, they can do that. The start over option would clear out all previously generated code and start from scratch.