WPS: Fixing the `getTimeData` (time-slider) process.

Changes:
 - Missing output format argument of the `execute()` method.
 - The use of `text/plain`, although still used by the time-slider,
   is deprecated and should be replaced by the `text/csv`.
   Before this happens support for the `text/plain` legacy format
   is needed not to break the current EOxClient installations.