rosdep init if rosdep not initialized

[pariaspe]

$ ~/as2_ws$ rosdep update
ERROR: no sources directory exists on the system meaning rosdep has not yet been initialized.

Please initialize your rosdep with

    sudo rosdep init

[pariaspe]

If rosdep init sources are already initialized, a warning shows up:

$ sudo rosdep init
ERROR: default sources list file already exists:
    /etc/ros/rosdep/sources.list.d/20-default.list
Please delete if you wish to re-initialize

[pariaspe]

Done in #36