DGN v7 file support

[paolo-caroni]

The most used CAD files are:
dwg;
dxf;
dgn.

The old Microstation DGN v7 and AutoCAD DXF files have public specification.
There is a free library for read and write DGN v7 files (http://dgnlib.maptools.org/) released under X11/MIT license.

Would you consider to add support for DGN v7 files?

[dhombios]

IGES support would also be a nice feature as most open source 3d mechanical CAD programs support this file format (and step) but they are usually unable to open DXF files