Refactor into modules and improve documentation

[daemontus]

Right now, we only have one big module where everything resides. We should split this up into sub-modules. These sub-modules can loosely follow the structure of sections/sub-sections in the original SBML specification.

In terms of documentation, ideally, we should have a reference to the core SBML specification in the documentation for each SBML wrapper struct. We could even copy the relevant content of the specification in each docstring. Furthermore, it is possible to include images in the generated documentation. Maybe we could include the class diagrams from the official specification document where applicable. This would be really nice.