Fix single mesh multipart import

[jozefdobos]

With the latest multipart updates, single mesh models (eg a test cube) won't show any geometry in the GUI.

[carmenfan]

this is the multipart bug (3drepo/assimp#1) if you turn off multipart on config it should work.

Do open it again (and give me the sample file) if it doesn't. Closing it due to it being a duplicate of the same issue (and there's nothing to be changed in bouncer, it's all in assimp)