Bug 15260: Fix calculation of add amount

Bug 15260: Fix calculation of add amount

In a sequence of closed days, we should take into account the nature
of each closed day as we encounter it in order to calculate the amount
to add to reach the next potential closed date. We are now doing this.

Sponsored-by: Cheshire West and Chester Council
Sponsored-by: Cheshire East Council
Sponsored-by: Newcastle City Council
Sponsored-by: Sefton Council
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>