EL9 installation

UPD 2024.07.11

unwipping because I think it should be merged asap: EL7 is not available anymore at DESY NAF WGS, and using just the installation script from this branch does not allow installation with one command, because xfitter-install' pulls the xfitter code from master, while changes needed also in cmake/Findyaml-cpp.cmake'

Spotted by @sataheri and @kklimek

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EL9 installation tested on DESY EL9 machines

yaml-cpp version needs to be changed

Currently some tests fail: profilerLHAPDF (different numbers of eigenvectors), chi2scanMTOP

PineAPPL installation not tested