Use External ZOFU if present

[kprussing]

A user may have a copy of ZOFU installed in a known location. This change set allows the user to specify ZOFU's installation details according to its find_package script and it will respect that. A copy of the find_package script is included for general usability since ZOFU does not install its find_pacakge script by default. This will fall back to fetching and building ZOFU if the user specified library is not available.

Closes #31

[kprussing]

This is probably not a perfect solution. ZOFU itself does not install its find_package script so I had to include a duplicate. However, the find_package script can report false positives. It may be worth while to update ZOFU's cmake system, but that is a separate topic.