Pushes temporary variable onto visualisation CQL array

There is a temporary variable created for use in an if check, possibly
for performance reasons, however the push to array call used the method
call again, this commit uses the temporary variable.
Removes memory leak from SubsetPanel

Further fix for issue #943 whereby the memory leak introduced in the
earlier fix has been removed. Track instances of FilterGroupPanels
against layers in the SubsetPanel and prevent FilterGroupPanels from
removing and re-adding filters.
Rename variable from fgps to filterGroupPanels
Merge pull request #979 from aodn/issue-943-b

Issue 943