not sure if you wanted to get rid of `Model` too, happy to revert that one

Maybe should *not* be defined by SBML.jl, rather by Base or some other standard
package, but we need it nevertheless. Exporting it means that other packages
will have problems with their own redefinitions of Maybe because name
conflicts. So just do not export it, leave others to redefine it or manually
copy it out, using `const Maybe = SBML.Maybe`.

St. Elmo authored Mar 14, 2021 and Miroslav Kratochvil committed Mar 15, 2021

Co-authored-by: Mirek Kratochvil <miroslav.kratochvil@uni.lu>