Specify a UUID version for GlobalIds

Description of the proposal:

Please see here for UUID versions: https://en.wikipedia.org/wiki/Universally_unique_identifier#Versions

Also, this is related to #8

I am not currently aware of a UUID version being specified in the docs. This can lead to differing expectations.

I would personally prefer maximising randomness. If we are going to have BIM models of entire cities, we want to prevent collisions.

Describe how it contributes to the objectives set in https://github.com/buildingSMART/NextGen-IFC/wiki/Towards-a-technology-independent-IFC:

What do we win:

Consistency.

If UUID4 is chosen, then randomness too, so we can have the world in a model :)

What do we lose

Inconsistency.

Some devs need to revisit their code.

Schema impact:

None really.

Instance model impact:

None really.

Backwards compatible:

Yep.

Automatic migration possible:

Yep.

Additional implications:

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Note that not all points need to be satisfied!
Backwards compatibility and file size are not concerns.