attempt to improve hdf5 loading performance:
* let fetcher threads sleep until specified currentTimeMillis
* check interrupted before actually loading from hdf5Reader. This ensures
  that only the current fetcher load is completed when fetchers are put to
  sleep, even if fetchers have been queued in the hdf5Reader synchronized block.
* smaller iobudget, only 3 rendering threads
wip: prefetching cells before rendering
more work on prefetcher
correctly handle interpolation schemes in prefetcher.
do not allocate new arrays for reordering hdf5 dimensions.
finished prefetching strategy:
* clean up Prefetcher (previously PlayWithGeometry)
* wait/notify instead of letting Fetcher threads sleep
* remove debug prints
* clean up IoTimeBudget
* keep references to entries accessed in current frame
  such that they cannot be cleared from the cache prematurely.
Added "current" and "active in fused" buttons for groups in VisibilityAndGroupingDialog.