PDB Parsers should provide an elements attribute

[AmeyaHarmalkar]

Elements attribute in PDBParser

Related to #2423, #2553 and #2630, the PDBParser does not provide an elements attribute. Primarily have an attribute to define the element of an atom as per the periodic table.

>>> import MDAnalysis as mda
>>> from MDAnalysis.tests.datafiles import *
>>> u = mda.Universe(PDB)
>>> u.atoms.elements
array(['N', 'C', 'C', 'O', 'C', 'Cu', 'Fe', 'Ca'], dtype=object)

Elements class exists and enforcing elements would be needed.

[AmeyaHarmalkar]

@IAlibay tagging you here just to bring it to your notice