class THnSparseT<TArrayD>: public THnSparse

 Templated implementation of the abstract base THnSparse.
 All functionality and the interfaces to be used are in THnSparse!

 THnSparse does not know how to store any bin content itself. Instead, this
 is delegated to the derived, templated class: the template parameter decides
 what the format for the bin content is. In fact it even defines the array
 itself; possible implementations probably derive from TArray.

 Typedefs exist for template parematers with ROOT's generic types:

   Templated name        Typedef       Bin content type
   THnSparseT<TArrayC>   THnSparseC    Char_t
   THnSparseT<TArrayS>   THnSparseS    Short_t
   THnSparseT<TArrayI>   THnSparseI    Int_t
   THnSparseT<TArrayL>   THnSparseL    Long_t
   THnSparseT<TArrayF>   THnSparseF    Float_t
   THnSparseT<TArrayD>   THnSparseD    Double_t

 We recommend to use THnSparseC wherever possible, and to map its value space
 of 256 possible values to e.g. float values outside the class. This saves an
 enourmous amount of memory. Only if more than 256 values need to be
 distinguished should e.g. THnSparseS or even THnSparseF be chosen.

 Implementation detail: the derived, templated class is kept extremely small
 on purpose. That way the (templated thus inlined) uses of this class will
 only create a small amount of machine code, in contrast to e.g. STL.

This class is also known as (typedefs to this class)