Explain requirements

gudhi-devel/src/Simplex_tree/concept/SimplexTreeOptions.h

Lines 18 to 19 in 2e94f63

    
           /// Must be a signed integer type. It admits a total order <. 
        
           typedef VertexHandle Vertex_handle;

It would be convenient if we added a comment (it doesn't have to show in the doc) explaining why it needs to be signed, so we can easily figure it out a few years later when we have a use case for an unsigned type.

This is actually a general comment, with Vertex_handle as an example. If there are other non-obvious requirements in other places...

	/// Must be a signed integer type. It admits a total order <.
	typedef VertexHandle Vertex_handle;