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.
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.