fix in case dreiding particle type name only has 2 characters (Zn for example)
fix in case dreiding particle type name only has 2 characters (Zn for example)

mefortunato/pysimm,plin1112/pysimm,polysimtools/pysimm,plin1112/pysimm,mefortunato/pysimm,polysimtools/pysimm