Cmake veccore style
This will allow to
- change backends after install as in VecCore
- use project from subdir in user project
Since this can be used from source I propose to add it to geantV in the form of
- git submodule (this will allow us to manage versions easily) or
- download it when we generate cmake file for geantV(IMO not very effective)
Including it inside geantV as a builtin will simplify introduction of this dependency to geantV CI system