projman #18406 begin-after-end-previous does not work for groups [open]

begin-after-end-previous does not work for groups : currently, the "begin-after-end-previous" condition takes the next real task. Hence, it can not point to a group which should be possible. In particular, if we take the "begin-after-end-previous" condition for a task with subtasks, it won't schedule:

File " projman/projmanedit/gui/editors.py", line 74, in on_button_schedule_start_clicked
  self._schedule_project(sol_max, max_time * 1000)
File " projman/projmanedit/gui/editors.py", line 79, in _schedule_project
  self.scheduler.schedule()
File " projman/scheduling/csp.py", line 316, in schedule
  self.project.add_schedule(activities)
File " projman/lib/project.py", line 129, in add_schedule
  self.update_caches()
File " projman/lib/project.py", line 167, in update_caches
  cost = self.get_task_total_cost(task, task.duration)
File " projman/lib/project.py", line 391, in get_task_total_cost
  costs = self.get_task_costs(task, task_tot_duration)[0]
File " projman/lib/project.py", line 408, in get_task_costs
  if 0 < task_tot_duration % (1./self.factor) < (1./self.factor):
TypeError: unsupported operand type(s) for %: 'NoneType' and 'float'
prioritynormal
typebug
done in0.17.0
load1.000
load left1.000
closed by<not specified>