Filtering on EventsFile

Add filtering capabilities to EventsFile object.

For example:

>>> segmentation.startswith('C_GPH', 'C_RS')
[<EventWindow> C_RS_6C1 (2032-06-20 -> 2032-06-21):
  - name: C_RS_6C1
  - t_start: 2032-06-20T12:37:11.000Z
  - t_end: 2032-06-21T12:37:11.000Z
  - subgroup: 
  - working_group: WG2,
...]