Adding a .mailmap file to the top level of the repo

This file helps avoid duplicate/redundant entries in the output of
'git shortlog -s'

.mailmap

@@ -0,0 +1,6 @@
+John W. Peterson <jwpeterson@gmail.com>
+John W. Peterson <jw.peterson@inl.gov>
+Roy H. Stogner <roystgnr@ices.utexas.edu>
+Roy H. Stogner <roystgnr@minerva.(none)>
+Cody Permann <codypermann@gmail.com>
+David Knezevic <david.knezevic@akselos.com>