Object conversion refactor

[tsvilans]

Object conversion refactor

Conversion functions are written more modularly, since code was being repeated, and the creation of objects and object data was too intertwined.

detailed explanation

• Object conversion now happens outside of main read_3dm() function
• Object and object data creation are separated. Object creation is now done in one place, instead of in different places (import_curve and import_render_mesh)
• Object data conversion functions now return the Blender object data
• !!! This changes several functions !!!

[lfertig]

looks good to me. nice to have it all tidied up a bit! once you have it merged I'll adapt my code, I have to wait for the next rhino3dm pip package anyway...