margin_threshold metadata property not set correctly if margin_order parameter is used in generation

[smcguire-cmu]

In margin generation, if the margin_order parameter is set, then the margin_threshold parameter is ignored and the margin is generated using pixels of the margin_order. But the margin_threshold property in the properties file is still set based on the ignored margin threshold. It would be useful to set this as the minimum pixel distance for the margin order.