Add non-sparse, n-dimensional histogram THnD, THnF, THnI...

THn and THnSparse now both derive from THnBase, providing the common interfaces and algorithms.
Extend stress suite to cover THn, too.
THn uses TNDArray for bin storage, an n-dimensional array. It provides an interface to extract an element by a series of operator[]:
Double_t value = arr[0][2][1][5];


git-svn-id: http://root.cern.ch/svn/root/trunk@42694 27541ba8-7e3a-0410-8455-c3a389f83636