Fix critical bug in GenEvent::remove_vertex()

Fix critical bug in GenEvent::remove_vertex().

The call of remove_vertex() should reassign the attributes of the vertex with id lower than that of the removed vertex. This is done with a loop over the vector of attributes that require this change. However, the later vector is created from a map of attributes and therefore is sorted ascending. This means that the current iteration deletes the attribute reassigned on the previous iteration. To avoid that it is necessary to loop in another direction, i.e. sort the vector descending.