This cleans up the handling of ElementLinks in the MET association code, particularly for leptons which use these links to access associated tracks, clusters, or PFOs. In some cases this is just code style improvements for readability, but it also adds a check for invalid links in cases where none previously existed. This should fix crashes with cryptic error messages in edge cases, e.g. where taus contain tracks which have been thinned from the input file. In cases where an error should be thrown, they will now come with a more informative message.
Tagging @salderwe, who prompted this by locating such an edge case.