Macro for default compositions

[lukem12345]

PR #52 added support for default composition diagrams, in which variables sharing the same name are to be composed.

Currently, the names for the boxes in the returned RelationDiagram are explicitly given as a Vector{Symbol}.

However, we can make the user experience a little nicer by also providing a macro that uses the name of the Decapode object as the name to be used in the box.

So, we should support a macro that does such. Further, we could define a method for default_composition_diagram which just accepts a Vector of models, and uses default names Component1 etc. or Model1 etc.