Option to toggle import update or create behavior

[jesterKing]

There is currently one importer option that is unused (mainly as a template for real options): https://github.com/jesterKing/import_3dm/blob/master/import_3dm.py#L200

Introduce a boolean option that controls whether existing objects are updated, or whether they get duplicated. This will mean that a Blender file could contain multiple objects that have the same Rhino object ID. Assume that for the update scenario only the first object found gets updated, others aren't handled.

The option should be called Update data, with as description When existing data is found with the same ID as in the 3dm file the existing gets updated (or better wording).

The option should be On by default.

[ghost]

hi, awesome work.

how would one go about implementing this?

thanks in advance.

EDIT: I've messed around with fair success, but in turn wrote a different addon with my own idiosyncrasies, thinking of creating a PR/Discussion to tackle a couple of improvements on this one (don't want to publicly maintain an entirely different project and scatter users)