Minor cleanup
Added tuple-get interface for array_md.

Added "get" non-member function templates for "array_md."  Added
specializations of the "std::tuple_size" and "std::tuple_element" class
templates for "array_md."
Added ordered-comparison operators to array_md.

Added operators < > <= >= for "array_md."  Tweaked some comments.  Added
check to "std::tuple_element<array_md>."