• Use lru_cache to speed up get_el_sp by 400x (@v1kko).
• Related to lru_cache of get_el_sp, Species.properties is now deprecated in favor of setting Species(spin=5). The rationale is
  that spin is the only supported property for Species anyway. Species and DummySpecies is now mostly immutable, i.e., setting specie.spin = 5 have no effect. This is as intended since the first version of pymatgen.