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.
Refactor empty() check within single if statement
Merge pull request #849 from ORNL-BTRIC/GridView

Update system() implementation in Case and Walkin