Make `_atom_type.element_symbol` the enumeration default on some data items

[rowlesmr]

Will close #401

Dataitems done in this commit:

• atomic_number
• atomic_mass
• radius_bond
  • Shouldn't these be ion-specific, or isn't that the intent here? cf charge vs oxidation in #378
• length_neutron
  • This will be the topic of another PR
• dispersion_real_cu, dispersion_imag_cu
• dispersion_real_mo, dispersion_imag_mo

Dataitems not being touched:

• electron_count
• ion_to_element
• cromer_mann_a1 -- cromer_mann_c
• hi_ang_fox_c0 -- hi_ang_fox_c3