exporting custom attributes in Alembic to Blender

We use the setattribtypeinfo function to force an attribute to be a color type. Just like Cd usually is.

This will probably streamlined and not necessary in future versions of Blender, but it seems like it is still necessary atm (Jan 2023)

using a point wrangle, set to detail (so it only runs once)

 setattribtypeinfo(0,"point","attributename","color");

here I'm forcing an attribute called attributename to be recognised as a color type & not just 3 floats.
I'm using the rop_alembic output to write the abc file.