Wrijupan Bhattacharyya / ctools

…no selection was performed)

The processing for empty event files is skipped. The file is simply written out empty again.

…(see #1360 for details).

…)-function of individual ctools

…ts the intervals, leading in the worst case to an empty event list if no overlap exists.

…d scave() methods to ctool interface. Add read_ahead() method to handle reading of task parameters at the beginning of the task execution. Remove all m_read_ahead members from the derived classes.