[CMake] project_name and var as full uppercase or lowercase but not camel case

[clementval]

When using find_package, we have to use Serialbox and not serialbox or SERIALBOX. I find it confusing and also for other variables used later.
When using the exported package information of serialbox2, we have to provide the directory with -DSerialbox_DIR=<path-to-cmake-folder-in-install>.
In my opinion this is not quite friendly and would be better with SERIALBOX_DIR= or serialbox_dir=. This is picky change but then necessary 😄

I would gladly make the change if you guys agree on this. @havogt @phmarti ??

[havogt]

I think I don't agree as this is (unfortunately) the standard CMake pattern.