from Axel: THnSparse only saves the filled part of its last chunk. When...

from Axel: THnSparse only saves the filled part of its last chunk. When appending bins to a read chunk ensure 
           that we allocate the full chunk size. 
           This is done using a new, non-persistent data member that keeps track of the chunk's current allocation 
           state.


git-svn-id: http://root.cern.ch/svn/root/trunk@23881 27541ba8-7e3a-0410-8455-c3a389f83636