ENH: Add raw cif labels

As it is now, the CIF reader only uses the _atom_site_label tag to extract element names as an alternative to _atom_site_type_symbol. However, the _atom_site_label information can be relevant - for example, it can identify by name crystallographic sites of interest. Here I allowed for this information to get stored in an array called cif_site_labels if the user so asks by setting save_raw_labels=True when calling the I/O reader. This required also some changes to xtal.py in order to keep track of which site is which when expanding the symmetry operations. Right now the functionality is only available if there are no fractional occupancies, as that would make things more complicated. Ideally that could be overcome too but it would need some more sophisticated solution.

Tagging @jensj and @yashasvi-ranawat as the topic may be relevant to them too. Thanks!