Print warning if no natural isotopes when using add_element and wrote unit test (#2938)

Co-authored-by: Catherine Yu <cathyy@umich.edu>
Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

openmc/element.py

@@ -1,4 +1,5 @@
 import re
+import warnings
 
 import lxml.etree as ET
 
@@ -123,6 +124,10 @@ class Element(str):
         # Get the nuclides present in nature
         natural_nuclides = {name for name, abundance in natural_isotopes(self)}
 
+        # Issue warning if no existing nuclides
+        if len(natural_nuclides) == 0:
+            warnings.warn(f"No naturally occurring isotopes found for {self}.")
+
         # Create dict to store the expanded nuclides and abundances
         abundances = {}