Fix T94516: OBJ/MTL Material Roughness imported and exported inaccurately.

The addon would assume an OBJ range of [0.0-900.0] for the Ns value,
when it actually is supposed to be [0.0-1000.0].

WARNING: This is introducing a slight incompatibility (value shifting of
the roughness parameter) with older OBJ files exported by Blender.