YCM is no longer consumed as a soft dependency

[PeterBowman]

As a result of #55, we are removing the usual bootstrap mechanisms that allowed YCM to pull itself from the web during the CMake configuration step (cd build && cmake ..). This means that, from now on, YCM is to be considered a hard dependency on all non-superbuild repos it was previously used in, that is, everything that doesn't follow the scheme <robot>-main (e.g. teo-main).

If you are using any of the affected repositories, please download and install latest YCM (0.8.1 at the time of writing) from https://github.com/robotology/ycm. Make sure that your CMake version conforms to the minimum requirements (on Trusty, install the cmake3 apt package in order to get the CMake 3.5 backport). Check Install YCM in case of trouble.

cc @roboticslab-uc3m/roboticslab.

[PeterBowman]

Related: YARP is probably going to follow this path and require a stand-alone YCM installation (robotology/yarp#2078).