improve automatic variable description

This MR adds a better description of the h5 variable content for PFlow jets and VR track jets, as well as a dump of all variables that the dumper can dump (pretty meta...). It also includes documentation of the GNN specific variables.

Further, it includes a fix of the README where a line was duplicated and fixes the indentation in the output of the dumper-docs-dumper (closes #19 (closed)).

Please note that the GNN variable qOpUncertainty is updated to qOverPUncertainty.