SOLID: removed dxf_solid_write_lowlevel().

Signed-off-by: bert <bert.timmerman@xs4all.nl>
SOLID: corrections in the Doxygen file headers.

Signed-off-by: bert <bert.timmerman@xs4all.nl>
SOLID: use DxfFile struct.

Signed-off-by: bert <bert.timmerman@xs4all.nl>
SOLID: use DxfFile struct.

Signed-off-by: bert <bert.timmerman@xs4all.nl>