improve LHE event counting
Current way of Gen_tf to count events in LHE files pulls the full files into memory at once, which may bring jobs close to boundaries imposed by e.g. the grid if files are large.
Previously, @zmarshal had fixed the issue e5081205
However, when fixing an unrelated issue, @cgutscho brought back the old behaviour aa3aceb0
This patch effectively restores Zach's patch, while also working on LHE files that have extra bits after "<event" tags
Net effect is about 1GB less memory used in a job that showers MG FxFx Vjets files in Pythia8.