Update dREL evaluation method of the ATOM_TYPE category

[vaitkus]

The values of _atom_site.type_symbol should be used first when determining the values of _atom_type.symbol since the assigned types might differ from those derived from atom labels.
Since the dREL method for _atom_site.type_symbol data item also derives the values from the values of _atom_site.label, the updated dREL will still yield the same result in cases when the values of _atom_site.type_symbol are not explicitly defined.

[vaitkus]

Also, I just realised that there might be a need to redesignate the entire method from 'Evaluation' to 'Definition' as the type symbol derived from the atom label is not required to be exact (see issue #452).
This would apply to both the updated code and the old variant.

[jamesrhester]

I don't think this method needs to change to Definition, but the ATOM_SITE method should.