The new policy is to always inherit number format of a formula cell
from its reference unless the cell already has an explicit number format
set.

Also to avoid recalculating formula cells on load just because they have
the 'General' number format.  This leads to excessive re-calculation of
formula cells upon load even when the cells already have results cached.

Change-Id: I28128d3fef296e09e62bea72e8aab75de9876239