Add weights to the Workspaces

The weights are taken to be the weight of the final cut level,
regardless of whether the event ends up in the denominator or numerator.
Fix false rejection bug in ZDefinitionWorkspace

In a previous commit (16e341b6afd0304253d439f681a0bc318fda7262) a bug
was introduced that incorrectly rejected all events with more than -1
vertices (that is, every valid events). This commit fixes the bug by
reverting the one line that caused the problem.