_create_xde in export_step does not accept color as string

[jwagenet]

This fails for AttributeError: 'str' object has no attribute 'wrapped'.

part = Box(1, 1, 1)
part.color = "black" # or "#000000"
part.label = "box"
export_step(part, part.label + ".step")

Using Color instead of string works, however string colors should be supported everywhere if supported anywhere.

part.color = Color(0, 0, 0)

[jwagenet]

Related to and likely solved by implementing #856