Calculating total surface area/capacity.

[iandriop]

As an extreme case we could have a location that does not contain any rooms yet or a location whose rooms do not have a surface/capacity value. In that case we should let the user know that this information is absent from some rooms. For example supposing that a user has 3 rooms with capacity 100, 0, 0. In that case the total capacity is quite deceiving if we don't let him know that the capacity information is missing from some rooms.