Slot Viewer: Misc. small fixes, incl. drop of AggSeriesSlot time sync problem.
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.2

Miscellaneous fixes:

(1) For the Slot Viewer application, in SlotQtDlg::displayCompressRecomp_needed
()'s call to SlotDataTableModel::refreshDisplayCompression(), specify 
'stableRequired' to prevent the cell selection to be modified.  The cell 
selection was being set to the DateTime spinner's DateTime (row), e.g. after 
adding slots to the viewer.  That wasn't desirable.

(2) Dragging an AggSeriesSlot to the viewer (but not when adding the 
AggSeriesSlot to the viewer in the more "conventional way") -- with an 
AggSeriesSlot having columns with different initial timestep DateTimes -- 
was showing a time synchronization problem.  (The series values weren't 
showing up in the correct timestep rows).  This was resolved with this change 
in method: bool SlotQtDlg::slotViewer_slotDragDropped():

   // Note [Phil, 7-2017, RW 7.2]: This was tested with an AggSeries Slot
   // with the first column starting later then the subsequent columns.
   // The SlotColRef really does need to be constructed here as the
   // overall slot for this to work, i.e. not with the column specification
   // of zero.
   //-- SlotColRef scRef (slotPtr, 0);

   SlotColRef scRef (slotPtr);

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