Add option to allow the collected metrics to be printed to stdout

[darvg]

The print format looks like this:

INFO:ros_cross_compile.data_collector:seconds data: Datum(name='cross_compile_end_to_end-time', value=32.94736960809678, unit='seconds', timestamp=2934190.857417992, complete=True)

Update:

The print format now looks like this:

-------------------------------- Collected Data --------------------------------
================================================================================
2020-07-22T01:02:00.769065 |                 gather_rosdeps-size: 224022065 bytes

2020-07-22T01:02:00.769074 |                 gather_rosdeps-time: 11.60 seconds

2020-07-22T01:02:00.876605 | create_workspace_sysroot_image-size: 842461430 bytes

2020-07-22T01:02:00.876614 | create_workspace_sysroot_image-time: 0.11 seconds

2020-07-22T01:02:21.702636 |      run_emulated_docker_build-time: 20.83 seconds

2020-07-22T01:02:21.703046 |                     end_to_end-time: 32.55 seconds

Resolves #211