Implements runtime fall back for simplification of unit string suffix

Compile time simplification of suffix string involves recursion for
partial specialization template that is not supported by the current
C++ standard. The library comes now with both compile time and
runtime support for suffix formating. Static Suffix() method returns the
compile time form (not simplified in all cases) while GetSuffix()
virtual method returns the runtime form (simplified in all case). Both
methods take into accound "kg" special case.