Update Dataset class
Update Dataset class

- Replace staticmethods _to_datetime and _to_timedelta with functions
  from typhon.utils
- Extend special chars and use them in generate_filename
- Extend magic methods and let them use filters
- Update copy method
- Fix bug in _get_searching_dirs that excludes path because their time
  coverage was not regarded.
- Use filters correctly and with regex now

atmtools/typhon,atmtools/typhon