Require CMake 3.16 or newer

Update the required CMake version in anticipation the addition of the ROOT-based edm4hep output. This is not actually a hard requirement, but removes a number of warnings. 3.16 is chosen as that is currently required by ROOT itself. While GUINEA-PIG without edm4hep could use also older cmake versions, the added complexity of making this requirement optional does not seem worthwhile, as setting up CMake is easy ( instructions added to README )