BHoM_Engine: in Create.RandomObject(), restore the functionality that instantiates Fragments

[alelom]

Description:

In this commit:
41e1085

we previously removed the functionality that instantiates Fragments from Create.RandomObject(). This was due to a bug that made it throw an exception when fragments were encountered.

We recently tested this functionality again and this problem does not appear to happen anymore, so the functionality can be restored.

Restoring this functionality would allow us to create Random Fragments again, including e.g. Materials, which is useful for testing purposes.