C++ library: Add class owning memory for a single mutable struct

[VeaaC]

flatdata supports sorting single structs in an archive (usually headers). Currently those are generated/written in C++ by creating a ``flatdata::Vector< T > v( 1 );```. Adding a class that represents this concept more closely would be more intuitive/readable.

Proposal:

• Add a template< typename T > flatdata::Struct{ ... } for this concept

[boxdot]

Closing since it was resolved in #24.