Upgrade cmake requirement to 3.16
Currently we require cmake 3.9
It is nice to have not very strong requirements on cmake, so this we should discuss.
The advantage of cmake 3.16 is to have CMP0097 that allows to not-download git submodules of external projects (cxroot). In earlier versions of cmake those are always downloaded (or we may change that...) leading to some inefficiency (the submodule of interest are the corsika-data, which has many 100s of MB.
One part of the solution would be to upgrade the cmake on the CI containers. Because this causes inefficiency on our workflow, and we may ignore the inefficiency on user installations.