This commit adds a new data member (GridProperty<T>::m_defaulted)
that keeps track of whether or not a particular data item has been
implicitly assigned from the input deck. GridProperty::setDataItem
assigns 'false', while the constructor initialises 'm_defaulted' to
true for all Cartesian cells.
The main objective is to be able to apply a post processor only to
those cells for which the input deck does not supply an explicit
value.
430cc9d7b2