FindHerwig3.cmake Update, 1.0 branch (2018.12.04.)

Made the FindHerwig3.cmake module a bit more robust.

Now it looks for both the "old" header, and the new one. (If either one is found, the Herwig include directory is considered found.)

Plus added a search for the library under a more traditional location than the one currently used by the Herwig installation. (If they should ever update to a more standard layout.)

This is a follow-up from !385 (merged), making that update a bit "nicer".