Improve docstrings

[ArnoStrouwen]

Such that makedocs warnonly = [:missing_docs, :cross_references] can be removed.

[paulflang]

How would I do that, without having to remove the docstring for, e.g. get_unidirectional_components? get_unidirectional_components is not exported, so the docstring is just intended for the programmer, not the user. I think I could try

Modules = [Foo]
Private = false
Order = [:function]

in api.jl.

The issue with the cross-references comes from Model sitting in SBML, not SBMLTk. Do you know how to get that working @ArnoStrouwen ? In any case, if Private=false it would not work, cause SBML.Model is private...

[ArnoStrouwen]

Maybe try not using cross references, use hard links to SBML?
For the docstrings I wouldn't worry right this moment, I think some tooling for that is coming later.