Todo
Fill overview page
zfit.func.BaseFunc([obs, dtype, name, params])
zfit.func.BaseFunc
TODO(docs): explain subclassing
zfit.func.SimpleFunc(obs, func[, name])
zfit.func.SimpleFunc
Create a simple function out of of func with the observables obs depending on parameters.
zfit.func.ProdFunc(funcs[, obs, name])
zfit.func.ProdFunc
zfit.func.SumFunc(funcs[, obs, name])
zfit.func.SumFunc