FindHDF5 optim, note cmake >= 3.20 needed for autobuild
- Note with error message that CMake >= 3.20 needed for autobuild of NetCDF (let us know if this is an issue, we could go back to CMake 3.15, with less reliability)
- add intel compiler optimization flags
- don't override CMAKE_INSTALL_PREFIX for nc4fortran used via CMake FetchContent