Switch to CMake 3.x

[ocommowi]

Switching medInria to CMake 3.x. This mainly corrects warnings that were not preventing medInria from compiling but were from packaging. It changes the way we declare versions in cmake, dumps minimal version of cmake to 3.0, plus some few things here and there.

This relates to many other PRs:

• I have a branch on dtk that sets its minimal version to CMake 3.x -> I didn't create a PR yet as we will probably switch to DTK 1 very soon
• DCMTK: Remove warnings and switch to cmake 3 dcmtk#2
• QtDCM: [FIX] - C-FIND - Query Retrieve Information Model qtdcm#5
• RPI: Switch to cmake 3 Inria-Asclepios/RPI#15
• TTK public: Switch to CMake 3.x Inria-Asclepios/TTK-Public#8
• medInria public: Switch to cmake 3 medInria-public#258

We'll have to change also private plugin repos later on.