Fix: Don't grow Integer Indexed Series Slots more than necessary [5964]
Bug Number: 5964
Release notes (y/n): Yes
For Release Nums: 7.2

Gnats 5964: Initialization Rule grows Integer Indexed Agg Series slot 
 unnecessarily.

See Patrick's added notes in that bug report.  The "geometric" growing of 
Integer Indexed Series Slots was intentional, but not actually necessary.
David's directions are: "Do as Patrick suggests, change it so integer indexed 
series slots only grow as much as necessary. There are never that many 
integer indexed series slots in a model, so the performance shouldn't be 
an issue."

The bool 'extendBasedOnRun' parameter (default: true) to Series<T>
extendToIncludeDates() has been replaced with this new enumerated type:

   typedef enum {
      ExtendSeries_OnlyAsNeeded = 0,
      ExtendSeries_BasedOnRun,   // extend at least to run period
      ExtendSeries_GrowGeometric // approx double length.
   } ExtendSeriesMode;

This parameter replacement has been applied also to higher level methods in 
SeriesSlot, SeriesSlotImpl, SplitSeriesImpl. In Series.cxx (the lowest level 
method), the default value is ExtendSeries_BasedOnRun.

The only place the new 'OnlyAsNeeded' value is used is in the place relevant 
to the behavior described by this bug: in SeriesSlot::assignStdValue(), 
where the 'extendBasedOnRun' condition had been based on the series slot 
NOT being an Integer Indexed Series Slot.  We now use this value for the 
replaced parameter:

   const ExtendSeriesMode extMode =
      _usersIndexByInt ? ExtendSeries_OnlyAsNeeded  // Gnats 5964
                       : ExtendSeries_BasedOnRun;

I did test these changes with regression tests (debug and release builds).

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