Expose ASPIF reader/writer via the API

[rkaminsk]

We already have an internal interface in the potassco library to read files in ASPIF format. We should provide a reader/writer via the API, too. The reader can use the same/similar interface as the Observer class.

For the 5.6 release there will be support for reading an aspif document from the command line. This must be the first file loaded. Afterward, the API can be used as usual. This will allow systems like clingcon or clingo-dl to load ground programs in aspif format. A dedicated reader might follow in later releases as there are no use cases for such a feature.