moved the demos to exercises

exercises/demos/2_data_flow/foreach_task.json

exercises/demos/2_data_flow/foreach_task.yaml

exercises/demos/2_data_flow/python_function_task.json

exercises/demos/2_data_flow/python_function_task.yaml

exercises/demos/3_files_and_commands/command_line_task.json

exercises/demos/3_files_and_commands/command_line_task.yaml

...ises/demos/3_files_and_commands/command_line_task_wf.json

...ises/demos/3_files_and_commands/command_line_task_wf.yaml

exercises/demos/3_files_and_commands/file_transfer_task.json

exercises/demos/3_files_and_commands/foreach_task_cmd.json

exercises/demos/3_files_and_commands/foreach_task_cmd.yaml

exercises/work/.gitkeep