[docs] Add citation

doc/doxygen/cantera.bib

@@ -217,3 +217,15 @@ @article{wagner2002
     url = {https://dx.doi.org/10.1063/1.1461829},
     volume = {31},
     year = {2002}}
+@article{kim2019,
+    author = {S.~Kim, and J.~Chen and T.~Cheng and A.~Gindulyte and J.~He and S.He
+        and Q.~Li and B.~A.~Shoemaker and P.~A.~Thiessen, and B.~Yu and L.~Zaslavsky
+        and J.Zhangn and E.~E.~Bolton},
+    journal = {Nucleic Acids Research},
+    number = {D1},
+    pages = {D1102--D1109},
+    title = {{PubChem} 2019 update: improved access to chemical data},
+    doi = {10.1093/nar/gky1033},
+    url = {https://dx.doi.org/10.1093/nar/gky1033},
+    volume = {47},
+    year = {2019}}

src/thermo/Elements.cpp

@@ -13,7 +13,8 @@
 namespace Cantera
 {
 
-/*! Database for atomic weights
+/**
+ * Database for atomic weights.
  * Values are used from CIAAW. Atomic weights of the elements 2017
  * when a single value is given. Available online at
  * http://www.ciaaw.org/atomic-weights.htm
@@ -36,13 +37,9 @@ struct atomicWeightData {
     double atomicWeight; //!< Element atomic weight in kg / kg-mol, if known. -1 if no stable isotope
 };
 
-/*! Database for named isotopic weights
- * Values are used from
- * Kim S, Chen J, Cheng T, Gindulyte A, He J, He S, Li Q, Shoemaker BA,
- * Thiessen PA, Yu B, Zaslavsky L, Zhang J, Bolton EE. PubChem 2019
- * update: improved access to chemical data. Nucleic Acids Res. 2019
- * Jan 8; 47(D1):D1102-1109. doi:10.1093/nar/gky1033.
- * [PubMed PMID:30371825]
+/**
+ * Database for named isotopic weights.
+ * Values are used from Kim, et al. @cite kim2019.
  *
  * units = kg / kg-mol (or equivalently gm / gm-mol)
  *