Renamed 'TimeFilter' to 'DateFilter', for consistency.  Removed unused filter type ('datetime') from server-side code.
Fixed indentation.
Re-added 'datetime' type - apparently it's a value which can possibly be sent back from geoserver.
Merge pull request #628 from aodn/date_time_filter_naming_consistency

Renamed 'TimeFilter' to 'DateFilter', for consistency.  Removed unused f...