build: Adding --extend-ws argument to control workspace parent

This adds `--extend-ws` as an argument to `catkin build`. This calls the function `load_workspace_environment()` at the beginning of the `catkin build` verb invocation which sources another workspace's environment before initiating the build process.

Calling `catkin build --extend-ws` also implicitly enables `--force-cmake`. This makes it easy to switch the parents of a workspace.

This also prints out the `$CMAKE_PREFIX_PATH` used by the workspace at the beginning of the build.

I'm not wedded to the argument name, if anyone has a better idea for it than `--extend-ws`, I'm all ears.