Feature: Add slicing modifiers to the generated 3MF

[kageurufu]

I would like to be able to output slicer modifiers from my FreePySCAD scripts, this would make writing generators for custom calibration routines really simple.

For example, my typical extrusion multiplier routine could be something like this:

minimum_flow = 95
maximum_flow = 102
flow_step = 1

scene().redraw(*[
    union(
        name="flow_test_{}".format(flow),
        layer_height=0.2,
        first_layer_extrusion_width='140%',
        bottom_solid_layers=0,
        perimeters=2,
        top_solid_layers=8,
        fill_density=0.25,
        fill_pattern='Rectilinear',
        print_extrusion_multiplier='{}%'.format(flow),
    )(
        cube(20,20,2),
        cut()(
            cube(20,28,0.2),
            linear_extrude(0.2, 0, 0)(text(str(flow), 6)).move(1,1),
        ).move(0,-8,0),
    )
    for flow in range(minimum_flow, maximum_flow+flow_step, step=flow_step)
])