Import only numeric answer and non-zero answer in case of formula #2126

We previously imported formula results which were 0, resulting in
an answer being created whereas we should not have any answer at all.
One of the reason for this is the use of SUM() in formulas which always
return a number, never NULL. Similarly formulas sometimes returns "".
Swwitch all computing to percentage value instead of absolute #2126

This is closer to original Excel files and favorize fomulas reuse.
But most importantly it allow easy import of existing formula from
Excel files.