Add the function TBuffer::AutoExpand to centralize the automatic

buffer extension policy.  This enable the ability to tweak it later
(for example instead of always doubling the size, increasing by
only at most 2Mb or take hints from the number of entries already
in a TBasket).


git-svn-id: http://root.cern.ch/svn/root/trunk@39548 27541ba8-7e3a-0410-8455-c3a389f83636