Add the option to export a specific group to the CLI

[phkahler]

System information

• SolveSpace version: Current
• Operating system: All

Expected behavior

It would be nice to add an option to export a 2d section from a specific group

Actual behavior

You can't do this from the CLI

Additional information

Someone has provided a patch on the forum, but it also involves excluding constraints from gcode. This should be reviewed, modified and included.