Data Exchange, RWObj_Reader - Facets with empty normals like 'f 1// 2// 3//' (#520)

Data Exchange, RWObj_Reader - Facets with empty normals like 'f 1// 2// 3//' (#520)

Removing an obsolete header comment in the PPM writer
Correcting the handling of facet indices when normals or texture coordinates are empty in the OBJ reader.
- In Image_AlienPixMap.cxx, the unused header comment is removed.
- In Graphic3d_Aspects.hxx, the default viewer settings comment is updated.
- In RWObj_Reader.cxx, additional checks are added
    to correctly parse OBJ face definitions with empty texture and normal indices.