CMake minimum version

OTB needs CMake 3.1. But ITK5 has a minimum version of 3.10 (ITK CMakeList). If we want to be compatible we will need to use a more recent version of CMake. 3.1 was released in December 2014. 3.10 in Novembre 2017. Moreover since ubuntu 18.04 it is CMake 3.10 that is distributed.

I think we should consider moving to 3.10. And that do not mean just bumping the number of version in the main CMakeList, there are a lot of stuffs that we will be able to simplify thanks to this.