Resolve "Let cluster_space.primitive_structure return structure with the actual elements"

Description

This is a simple fix of issue #273 (closed). I have to admit I am not sure that the order of the elements in cluster_space._get_primitive_structure().toatoms() and cluster_space._primitive_chemical_symbols but I thought @angqvist would be able to confirm this.

I also removed the now needless manual setting of elements in the primitive structures in the MC tutorial.

Closes #273 (closed)