The data values in a deck item can be in three different states, given
by the DeckValue enum in DeckItem.hpp. The three values are:
SET_IN_DECK : The value has been set explictly in the deck.
DEFAULT : The value was not present in the input deck, but a default
value has been supplied in the configuration and that value
has been set.
NOT_SET : No value has been set for this item; it was not explicitly
set in the deck and also not included in the configuration.
If you ask for DeckItem->value which is in state NOT_SET you will get an
exception. The method setInDeck() can be used to check if a value has
been set explicitly in the deck; the method defaultApplied() will check
if a default value has been applied.
Observe that the system for handling defaults is not really well suited
for multi valued data items, as it is only a scalar state variable. In
the case of multi valued data items both defaultApplied() and
setInDeck() might return true.
The purpose of this flag is to keep track of whether a keyword is
supposed to have only one element, i.e. scalar, or several. The
defaultApplied method only makes sense in the case of scalar items, this
method will now throw if it is called on a non-scalar item.
