Replace CpymadService by a simpler model Factory

Until to now, the package was missing a convenient way to instanciate models given a specific locator. This gap is now filled by the model.Factory type.

Furthermore, the CpymadService becomes unnecessary, since the instance management belongs in user code IMO and instance creation is performed by the conceptually simpler model.Factory.

*Note:* the first two commits are part of the pure-package PR which is not an essential dependency for this. It was just to avoid a merge conflict later on. But I can rebase it, if you reject the other PR.