Duplicate vertices checker makes export really slow

[LaurensJN]

Using a model with 94.000 vertices, the exporter takes 74 seconds when the exporter checks for duplicate vertices. Without checking, it takes only 1.4 second. That's 70x faster. For bigger models the checker will probably be even worse.

What probably makes it slow is that it checks (1) each coordinate individually if it's already in a list; (2) it does not use any rounding. Another method should be considered that can make it faster, maybe similar to the CJIO remove_duplicate_vertices method (see here). That method also uses some rounding that would make it faster. On the model that is exported without checking for duplicate vertices I tried the CJIO method (with 12 decimals) and it was <1s.

[LaurensJN]

I will try to propose a new method