Add top-level Git mailmap

Since 2003, this codebase has substantially grown thanks to the
input of several tens of contributors. Over two decades, commits
from the same author are associated with multiple variants of the
author line. In order to effectively review and audit contributions,
it is necessary to normalize this variant.

This commit attempts to do so by using the Git mailmap functionality
to associate all commits from the same individual to a single
First Name Last Name <email> form. When applicable, either the
University of Dundee or the Glencoe Software email address is used
as the canonical address.

.mailmap

@@ -0,0 +1,18 @@
+Aleksandra Tarkowska <[email protected]> <[email protected]>
+Andrew Patterson <[email protected]> Andrew J Patterson <[email protected]>
+Chris Allan <[email protected]> <[email protected]>
+Colin Blackburn <[email protected]> <[email protected]>
+Colin Blackburn <[email protected]> <[email protected]>
+Dominik Lindner <[email protected]> <[email protected]>
+Jean-Marie Burel <[email protected]> jburel <[email protected]>
+Jean-Marie Burel <[email protected]> jean-marie burel <[email protected]>
+Josh Moore <[email protected]> <[email protected]>
+Josh Moore <[email protected]> <[email protected]>
+Josh Moore <[email protected]> <[email protected]>
+Josh Moore <[email protected]> <jmoore@05709c45-44f0-0310-885b-81a1db45b4a6>
+Mark Carroll <[email protected]> <[email protected]>
+Melissa Linkert <[email protected]> <[email protected]>
+Rémy Dornier <[email protected]> Rdornier <[email protected]>
+Roger Leigh <[email protected]> <[email protected]>
+Sébastien Besson <[email protected]> <[email protected]>
+Will Moore <[email protected]> <[email protected]>