add png of ifc file read into 12d Model and updated readme

[LeeGregory12d]

Additions to IFC4.3 Unit test UT_SpatialStructure_4

png showing ifc file read into 12d Model using/not using IfcMapConversion Aim: demonstrate that the fc 4x3 file can be read in

Review requested

• check that the png is as expected
  @larswik @pjanck