Corrected spelling in test and exception messages

Signed-off-by: Dave Hickin <david.hickin@diamond.ac.uk>

src/factory/Compare.cpp

@@ -62,7 +62,7 @@ bool operator==(const Field& a, const Field& b)
             return A==B;
         }
     default:
-        throw std::logic_error("Invalid Field type in comparision");
+        throw std::logic_error("Invalid Field type in comparison");
     }
 }