new Coin3D package, plus several fixes to the global CMake setup
This MR creates a new Externals package for Coin3D. Coin3D is a 3D graphics engine used by VP1. Originally Coin3D was taken from LCG, but the version shipped with LCG is not compiling with GCC 6. To speed-up things, we created anew External package to work on the code and to patch it locally. When everything will be set up, we will ask the LCG people to apply the needed changes to the Coin3D source and we will go back using the version shipped with LCG. This fixes ATLASVPONE-347.
Also, this MR includes a few fixes to the global CMake setup, from @akraszna.
Edited by Riccardo Maria Bianchi