Prior to this change, when a cell was clocked, it was always computed
with prior values of referred-to cells. This had the effect, for example,
of making cells defined only with a base expression as a total of a column,
for example, to be out-of-date in that they would take on the value of
total of the prior values of the column, not the new current values of
the column. This behavior was very counterintuitive.
With this change, updates of cells that have no clocked expression are
delayed until after clocked expressions have been recomputed based on
prior values and their cells' current values have been updated. Then
the computations of those base-only cells use all of the new current values,
leaving the spreadsheet in a (maximally) self-consistent state.
