Introduce mailmap file to match committer name "Convex Function" to proper name

Unfortunately, git-log doesn't use this file by default.  Only git-blame
and git-shortlog do.

Thanks to Elias Pipping for the hint about mailmap.

.mailmap

+Benjamin Bykowski <benjamin.bykowski@rwth-aachen.de> Convex Function <329364@wright.mathepool.rwth-aachen.de>
+