Fix [5735]: Typing value into Edit Account dialog leads to errors and assertion failures.
Bug Number: 5735
Release notes (y/n): Yes
For Release Nums: 7.1.5 and 7.2

It's a little surprising this assertion failure hasn't shown up sooner.  When 
assigning a value to an Accounting Series Slot in the Edit Account dialog, 
the SlotGUIUtils::spanTimestepsInCol() function is called to ensure that the 
Series Slot's time range includes the timesteps being assigned.

That function calls SeriesSlots::spanDates(). For an Accounting Slot, it is 
necessary for the last timestep in the range being spanned is at least the 
end of the accounting period.  An assertion test confirms condition.

The fix was applied to that SlotGUIUtils::spanTimestepsInCol() function.  If the provided slot is an Accounting Series Slot, we ensure that the minimal interval on the slot's time series includes at least the end of the accounting period.

Some minor changes were made also to SeriesSlot.cpp to ease debugging (e.g. breakpoints on premature returns from methods).