Update system() implementation in Case and Walkin

This implementation uses std::find() to look for the case and returns
the refrigerationSystem. The old implementation didn’t break all for
loops when the system was found. It also checks for empty vector of
cases/walkins first.

@evanweaver I didn't compile this on my machine (didn't want to do a full build of this branch) but I can't see any reason why this wouldn't compile.