Allow to give CAD files with relative paths similar to importing xml files (DDCAD)

[armin-ilg]

Hi

As discussed with @andresailer in key4hep/k4geo#273 (comment), it would be great if CAD files could be imported using DDCAD giving relative paths as it is done for xml files.
So instead of having to give an absolute path:

<shape type="CAD_MultiVolume" ref="${K4GEO}/share/k4geo/FCCee/IDEA/compact/IDEA_o1_v01/VertexSupport_o1_v01.obj" unit="mm"/>

I would like to give a relative path from the parent xml file:

<shape type="CAD_MultiVolume" ref="VertexSupport_o1_v01.obj" unit="mm"/>

which should work independently of the current working directory.

In the current DDCAD examples (e.g Check_Shape_MS3D_jeep.xml) there are always absolute paths given.

Let me know if I'm not clear, thank you!

Armin

[MarkusFrankATcernch]

See PR: #1175

[MarkusFrankATcernch]

Once you have checked that this issue is fixed by PR 1175, please close the issue.

[armin-ilg]

It's working, thank you!