Use system package if available

[Trophime]

I would like to know if it's possible to change the cmake so that to try to upload packages from the system before trying to build them from contrib. I wish to do that at least for:

• metis
• nlopt
• pybind11
• eigen3 (there are issues when using ParaView from debian archive which is build with eigen3 and the contrib eigen)

and eventually:

• mmg
• mongo stuff
  since I've got some packages for these also