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.