Add `override` keyword for all cisst libraries

[adeguet1]

For all ROS packages, we now suggest to use clang on Ubuntu 18.04 to support some more modern C++ required for the cisst/ROS bridges. One annoying side effect is that clang generates a warning for each overloaded method not declared with override. On a personal note, I started to use the keyword fairly systematically as it helps me find errors when declaring overloaded methods. Since override has been introduced in C++ 11 I assume we can safely add it to the cisst libraries.