Add offline file with a subselection of 10 events, reconstructed with the `jmuon` and `jshower` chain

Adds an offline file to <km3net-testdata>/km3net-testdata/data/offline with a sample of 10 events, reconstructed with both the jmuon and jshower reconstruction chains. The offline file corresponds to a gSeaGen electron-neutrino file within the energy range between 10 and 100 GeV from the v5 mass production. The triggering and reconstruction was done using Jpp v16.