class THnT<double>: public THn

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

 THn 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. The actual storage is delegated to
 TNDArrayT<T>.

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

   Templated name        Typedef       Bin content type
   THnT<Char_t>          THnC          Char_t
   THnT<Short_t>         THnS          Short_t
   THnT<Int_t>           THnI          Int_t
   THnT<Long_t>          THnL          Long_t
   THnT<Float_t>         THnF          Float_t
   THnT<Double_t>        THnD          Double_t

 We recommend to use THnC 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. THnS or even THnF 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)