Ubuntu 18.04 support about robotology-superbuild

[traversaro]

A possible problem is that the dependencies of libopencv-dev are quite massive, so it may make sense to actually only install the dependencies actually used. Unfortunatly the legacy header cvaux.h that is still used in YARP (see https://github.com/robotology/yarp/search?q=cvaux.h&unscoped_q=cvaux.h) is only contained in the libopencv-dev package in Ubuntu 18.04 (see https://packages.ubuntu.com/bionic/amd64/libopencv-dev/filelist), so I think we are constrained for now to just install the package libopencv-dev .

[traversaro]

I was finally able to test the robotology-superbuild on Ubuntu 18.04 (actually running on WSL).

I noticed that the only problem is that some deb packages in the dependencies have changed names:

Package libhighgui-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  libopencv-dev

Package libcv-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  libopencv-dev

Package libcvaux-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  libopencv-dev

E: Package 'libcv-dev' has no installation candidate
E: Package 'libhighgui-dev' has no installation candidate
E: Package 'libcvaux-dev' has no installation candidate

Apparently libcv-dev, libhighgui-dev and libcvaux-dev have been actually deprecated in favor of the libopencv-dev metapackage for a long time, so I think it make sense to just tell the users to install libopencv-dev on all the Debian and Ubuntu distributions that we support.

[traversaro]

Fixed by #80 , if there are any more problem please open other issues.