Add functionality to create/edit yaml model

[jvanhoefer]

@dilpath suggested, to have a small module, that allows the user to create/load and modify a yaml model, e.g. via

model = yaml2sbml.YamlModel()

model.add_parameter(...)
model.add_ode(...)
model.write_to_yaml(...)

(I.e. something similar to libsbml...) I think, this would be a really nice feature of our tool and should not be too much work to implement. One of the examples would even benefit a lot from having such a tool (so this also has a use case) .If you, @martamatos, do not have any objections, I would invest my next free day in writing such a thing :)