use Apr_10 fake matrix
bugfix SusySelection::isEventForHft: use selection flags, not event vars

For the events that pass the selection due to charge flip, the event
variables indicate that they are OS. The event selection flags do
report the correct value when the event is charge-flipped.
initialize hft trees on first event

This way we will avoid the warning for the 'missing samples' when
in fact there are no events. Fixes #49