Improve dxf driver

[perob]

Hi,

I have modified dxf driver for better support of drawings drawn in various object coordinate systems. Pull request is sent also.

Entities changed are: point, line, polyline, ellipse, spline and text.

Remove ocs transformations for: point, line and spline. Remove ocs transformations based on entity data: polyline, ellipse. Add anchor position for text entity.

Points, lines and splines are always in world coordinate system.

Polylines may be 2d (ocs, with or without elevation) or 3d (wcs) and this information is stored in polyline flag.

Ellipses are stored in wcs but calculations must be done in ocs. Added inverse transform to OCSTransformer class and updated ellipse handling accordingly.

Anchor position for text entities was added to style string.

Object coordinate system features of DXF format are rarely used so please comment if this feature is needed for dxf driver.

Screenshots of sample files show desired output in CAD program.

Thanks, bye

[perob]

ocs xy plane parallel to wcs

[perob]

ocs xy plane NOT parallel to wcs

[perob]

screenshot as drawn

[perob]

screenshot as drawn