Requested changes for PR

Modified data.py to align with OpenMC Python convention,
added more elegant check for element names

openmc/data/data.py

@@ -4,6 +4,7 @@
 import re
 from warnings import warn
 
+
 # Isotopic abundances from Meija J, Coplen T B, et al, "Isotopic compositions
 # of the elements 2013 (IUPAC Technical Report)", Pure. Appl. Chem. 88 (3),
 # pp. 293-306 (2013). The "representative isotopic abundance" values from

openmc/material.py

@@ -530,8 +530,9 @@ def add_element(self, element, percent, percent_type='ao', enrichment=None):
         # Allow for element identifier to be given as a symbol or name
         if len(element)>2:
             el = element.lower()
-            element = openmc.data.ELEMENT_SYMBOL.get(el,"empty")
-            if element == "empty":
+            if el in openmc.data.ELEMENT_SYMBOL:
+                element = openmc.data.ELEMENT_SYMBOL[el]
+            else:
                 msg = 'Element name "{}" not recognised'.format(el)
                 raise ValueError(msg)