Refactor in order to prepare for DateRangeFilterPanel implementation.

- renamed *Filter to *FilterPanel (to be more consistent with how other classes are named).
- calculating CQL on the fly (rather than caching in BaseFilter.CQL)
- various other cleanups.
Merge pull request #635 from aodn/filter_panel_refactor

Refactor in order to prepare for DateRangeFilterPanel implementation.